What top plainclothes copper did is rotten (6)
I believe the answer is:
rancid
'is rotten' is the definition.
(rancid can mean rotten or foul)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for rancid that I've seen before include "Like fat that's gone off" , "Stale, with an offensive smell" , "Tasting or smelling sour, butter say" , "Rank-smelling." , "(Of fatty foods) gone off" .)