Putrefy, decay (3)
I believe the answer is:
rot
This clue is a double definition.
'putrefy' is the first definition.
(I've seen this before)
'decay' is the second definition.
(rot can mean to decay)
(Other definitions for rot that I've seen before include "'Decay, decompose (3)'" , "Gradual decline" , "Collapse" , "Decay - nonsense" , "crumble" .)