Cook while in decay (5)
I believe the answer is:
roast
'cook' is the definition.
(I know that roast is a more specific form of the action cook)
'while in decay' is the wordplay.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'in' indicates putting letters inside.
'decay' becomes 'rot' (rot can mean to decay).
'as' placed within 'rot' is 'ROAST'.
(Other definitions for roast that I've seen before include "Large piece of oven-cooked meat" , "Carpet" , "Oven-cook, tease" , "Traditional dinner" , "Cook in closed oven" .)