Chemical to decay with lid taken off (5)
I believe the answer is:
'chemical' is the definition.
(I know that ester is a type of organic compound)
'decay with lid taken off' is the wordplay.
'decay' becomes 'fester' (I've seen this before).
'with lid taken off' means to remove the first letter (the top letter or 'lid' is removed).
'fester' with its first letter taken away is 'ESTER'.
'to' acts as a link.
(Other definitions for ester that I've seen before include "Chemical compound of acid and alcohol, found in trees" , "Reset (anag.)" , "Organic chemical compound" , "Compound; trees (anag.)" , "Organic compound, rhymes with court joker" .)