Bad thing the head of a certain police department did? (6)
I believe the answer is:
rancid
'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" .)