Rank, sour (6)
I believe the answer is:
rancid
This clue is a double definition.
'rank' is the first definition.
(I've seen this before)
'sour' is the second definition.
(I know that sour can be written as rancid)
(Other definitions for rancid that I've seen before include "Rank-smelling." , "Smelling or tasting bad" , "Sour like stale fat" , "Putrid" , "(Of fatty foods) gone off" .)