Gone off (6)
I believe the answer is:
rancid
'gone off' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rancid that I've seen before include "Smelling or tasting of stale fat" , "Unpleasantly stale" , "Like stale fat or oil" , "Stale, with an offensive smell" , "Turned - foul-smelling" .)