Rotten, foul (6)
I believe the answer is:
rancid
This clue is a double definition.
'rotten' is the first definition.
(I've seen this before)
'foul' is the second definition.
(I've seen this in another clue)
(Other definitions for rancid that I've seen before include "Smelling bad like stale fat" , "Smelling or tasting bad" , "Sour like stale fat" , "Stale, with an offensive smell" , "(Of butter) gone off" .)