John Gill Commentary Acts 23:21

"Do not thou therefore yield unto them: for there lie in wait for him of them more than forty men, who have bound themselves under a curse, neither to eat nor to drink till they have slain him: and now are they ready, looking for the promise from thee." — Acts 23:21 (ASV)

But do not you yield unto them
Or be persuaded by them, to bring Paul down from the castle to the sanhedrim; this must not be imputed to the young man's pride and vanity, in taking upon him to give advice to the chief captain; but to his great affection for his uncle, which moved him to entreat, rather than to direct him, for which he gives a good reason:

for there lie in wait for him more than forty men, which have bound
themselves with an oath, that they will neither eat nor drink till
they have killed him; (See Gill on Acts 23:12). (See Gill on Acts 23:13).

and now are they ready ;
to execute their designs, being met together, and lying in ambush in some place, between the castle and the place where the sanhedrim met:

looking for a promise from you ;
that when the sanhedrim should apply to him, he would promise them to bring Paul down according to their request; and for the making and performing of this promise, these men were waiting.