Crossword Genius

What top plainclothes copper did is rotten (6)

Ross

I believe the answer is:

rancid

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'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" .)

I've seen this clue in The Sun.
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