Decay, decompose (3)
I believe the answer is:
rot
I believe this is a double definition.
'decay' is the first definition.
(rot can mean to decay)
'decompose' is the second definition.
(synonyms)
(Other definitions for rot that I've seen before include "crumble" , "Decay, perish" , "Corruption" , "Drivel" , "Rubbish (colloq.)" .)