Bent police, ultimately: head is out of prison first (9)

I believe the answer is:

proneness

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'bent' is the definition.
(thesaurus)

'head is out of prison first' is the wordplay.
'head' becomes 'ness' (ness is a headland).
'is out of prison' becomes 'prone' (I can't justify this - if you can you should believe this answer much more).
'first' means one lot of letters go next to another.
'ness' after 'prone' is 'PRONENESS'.

'police ultimately' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.

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I've seen this clue in The Guardian.

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