Produce mostly stiff undergarment (7)
I believe the answer is:
harvest
'produce' is the definition.
(thesaurus)
'mostly stiff undergarment' is the wordplay.
'mostly' means to remove the last letter.
'stiff' becomes 'hard' ('hard' can be similar in meaning to 'stiff').
'undergarment' becomes 'vest' (vest is a kind of undergarment).
'hard' with its last letter taken away is 'har'.
'har'+'vest'='HARVEST'
(Other definitions for harvest that I've seen before include "Ingathering of crops" , "Cut down" , "Crop-gathering time" , "The fruits of tillage" , "Crop gathering" .)