Crossword Genius

Bad thing the head of a certain police department did? (6)

Ross

I believe the answer is:

rancid

I'm a little stuck... Click here to teach me more about this clue!

'bad' is the definition.
(I've seen this before)

'thing the head of a certain police department did?' is the wordplay.
I cannot really see how this works, but
'a' could be 'an' and 'an' is found within the answer.
'police' could be 'cid' and 'cid' is found within the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for rancid that I've seen before include "(Of fat/oil) stale and foul smelling" , "Rank-smelling." , "Stale, rank" , "Stale, with an offensive smell" , "(Of butter) gone off" .)

I've seen this clue in The Telegraph.
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