Criminal unwell down under (5)

I believe the answer is:

crook

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'criminal' is the definition.
(I know that criminal can be written as crook)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for crook that I've seen before include "Hooked staff" , "Professional criminal" , "Criminal - taken in hand by bishop" , "Swindler - crosier - ill (in Australia)" , "Something kinky" .)

I've seen this clue in The Sun.

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