Become very warm when in decay (5)
I believe the answer is:
roast
'become very warm' is the definition.
(I've seen this before)
'when in decay' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'in' means one lot of letters goes inside another.
'decay' becomes 'rot' (rot can mean to decay).
'as' put into 'rot' is 'ROAST'.
(Other definitions for roast that I've seen before include "Cook in closed oven" , "Honey-based drink" , "Berate - cook" , "Traditional Sunday lunch" , "Cooked meat; criticise" .)