Go off to collect a small piece of meat (5)
I believe the answer is:
roast
'piece of meat' is the definition.
(piece of roasted meat)
'go off to collect a small' is the wordplay.
'go off' becomes 'rot' (as in food going off).
'to collect' indicates putting letters inside (inserted letters are collected by others).
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'s'='as'
'rot' going around 'as' is 'ROAST'.
(Other definitions for roast that I've seen before include "Give a telling off to - cook" , "Large piece of oven-cooked meat" , "traditional meal" , "Feel hot" , "Meat to get hot" .)