Unable to sleep because of an all-night feast (5)
I believe the answer is:
awake
'unable to sleep' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for awake that I've seen before include "Conscious and (usually) open-eyed" , "Become active again" , "Unable to sleep" , "On the ball?" , "Rouse; vigilant" .)