What winds one up in firm's do? (5)
I believe the answer is:
feast
'do?' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for feast that I've seen before include "Banquet or religious festival" , "Celebratory meal" , "Big meal" , "Lavish meal" , "Huge nosh-up" .)