Suffering from decay (6)
I believe the answer is:
rotten
'suffering from decay' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rotten that I've seen before include "Nasty" , "Decaying or morally corrupt" , "Decayed, putrid" , "'Gone bad, decayed (6)'" , "Putrefied; punk leader" .)