Decomposing (6)
I believe the answer is:
rotten
'decomposing' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for rotten that I've seen before include "Falling to pieces from age or morally contemptible" , "Decayed or despicable" , "Decaying - quite putrid" , "Decomposed" , "Suffering from decay" .)