Leave mostly rotten meat (4)
I believe the answer is:
veal
'meat' is the definition.
(veal is a kind of meat)
'leave mostly rotten' is the wordplay.
'mostly' means to remove the last letter.
'rotten' indicates an anagram.
'leave' with its final letter removed is 'leav'.
'leav' anagrammed gives 'VEAL'.
(Other definitions for veal that I've seen before include "Calf meat" , "what the meat course may be" , "cuisine" , "meat, R [ROSE] maybe" , "Type of meat" .)