A standard meaning of stand by is accompany loyally. This is the only definition provided for the transitive use of that compound verb in Merriam-Webster:
transitive verb
: to remain loyal or faithful to : DEFEND
// stood by his decision
The line in question uses the phrase transitively: return stands by disdain. It can be explained: Do not follow a path to glory too hastily, because your return along that path will accompany disdain like a loyal companion. I.e., be circumspect when you seek recognition, because even if you get it, you are always at risk of losing your glory and encountering scorn instead.
A line-by-line reading of the quoted excerpt shows how this meaning is applicable here.
Who list his wealth and ease retain,
Himself let him unknown contain.
Those who want to retain an easy life should stay inconspicuous and not draw attention to themselves. This is easily relatable to the historical circumstances of Wyatt's times, and even specifically to his own life. Wyatt was born into comfortable circumstances, but was a relatively unknown quantity until he was singled out to be a diplomat for Henry VIII. Being close to the king was of course a great opportunity, but given Henry's mercurial temperament, also perilous. Many who had served Henry admirably, including Thomas Cromwell and Thomas More, ended up beheaded when they fell out of favor.
Wyatt himself came very close to meeting this fate. It is generally believed that Wyatt was a lover of Anne Boleyn, Henry's second wife, for whom he divorced Catherine of Aragon and precipitated the English Reformation. Wyatt had known Anne for several years prior to her marriage to the king. When Anne herself fell out of favor with Henry, the king accused her of adultery. Since adultery against the king also constituted treason, Anne was convicted of a capital crime. Several men rumored to be her lovers, including Wyatt, were arrested around the same time. Wyatt was in the Tower of London when Anne was executed, and it is said that this poem was written after he witnessed her beheading from a window in his cell.
This explains the next two lines:
Press not too fast in at that gate
Where the return stands by disdain:
Do not be too hasty to go through the gate where, when you return by it, you will be inevitably accompanied by scorn. This sentiment applies to Anne Boleyn just as much as it does to Wyatt. Anne was well-born and could have had the typical life of a noblewoman of her day: marriage to a courtier and a comfortable, if relatively obscure, life. Instead, she came to the king's notice. This got her the glory of being England's queen, but it was also her downfall. Having pressed ... in at that gate of fame and fortune, her return stood by disdain. She was called an adulteress and a whore when probably her only real crime was that she bore Henry a daughter instead of the male heir he longed for.
Wyatt narrowly escaped Anne's fate. Not only did he come very close to losing his own life; those convicted of treason also had their wealth confiscated, so his family would have lost their money as well. Wyatt's expert diplomatic skills, both personal and professional, allowed him to evade this outcome. He was able to regain the king's favor and was in fact not only released from the Tower, but also restored to his diplomatic missions on behalf of the king. His tumultuous experience informs the last line of this verse:
For sure, circa regna tonat.
The Latin phrase translates to Thunder encircles the throne. As the idea of being a loyal companion to disdain itself indicates, Wyatt is a master of irony. This line has two levels of meaning ironically played off against each other. The first straightforwardly continues the philosophical exhortation of the previous lines: if you achieve any kind of splendor, such as a throne, danger is all around you. The second refers to Henry VIII's mercurial temperament. Henry is metonymically the throne, and his raging temper the thunder.
From this historical and biographical explanation, we can understand why Wyatt counsels the reader to beware of seeking fame. People whom Henry favored could achieve great splendor, but only by risking their reputation, money, and lives. Generalizing from this specific situation, Wyatt says in this poem that it is better to be comfortable rather than to gamble that comfort for the sake of greater glory. The loss of that glory is almost certain, and disdain is always standing by.
