Rank or sour (6)
I believe the answer is:
rancid
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">rank</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">sour</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that sour can be written as rancid)</span><br/ ><br/ ></span>
(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" .)