Rank or sour (6)
I believe the answer is:
rancid
I believe 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 "(Of butter) stale" , "Fragmented bricks etc." , "Noxious" , "Smelling bad like stale fat" , "Like stale fat or oil" .)