Rot, decompose (5)
I believe the answer is:
decay
This clue is a double definition.
'rot' is the first definition.
(I know that rot is a more specific form of the action decay)
'decompose' is the second definition.
(I know that decompose can be written as decay)
(Other definitions for decay that I've seen before include "Go bad" , "'Decompose, rot (5)'" , "Rot, moulder" , "Deterioration" , "Rot, perish" .)