Crook in worthless circle? (7)

I believe the answer is:

villain

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'crook in worthless circle?' is the definition.
'villain' can be an answer for 'crook' (thesaurus). I'm not sure about the 'in worthless circle?' bit.

'crook in worthless circle?' is the wordplay.
'crook in' becomes 'ill' (I can't justify this - if you can you should believe this answer much more).
'worthless' becomes 'vain' (I've seen this before).
'circle?' is an insertion indicator.
'ill' going into 'vain' is 'VILLAIN'.

Can you help me to learn more?

(Other definitions for villain that I've seen before include "Evil character" , "Evil-doer" , "Wrongdoer" , "Bad person in story/play" , "Scoundrel" .)

I've seen this clue in the Sydney Morning Herald.

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