Criminal humanoid turned fine (5)
'criminal' is the definition.
'humanoid turned fine' is the wordplay.
'humanoid' becomes 'orc' (humanoid creature in fantasy genre).
'turned' shows that the letters should be reversed in order.
'fine' becomes 'ok' (synonyms).
'orc' written backwards gives 'cro'.
(Other definitions for crook that I've seen before include "Well, no", "Fence, perhaps", "Shepherd's staff", "Something kinky", "Twister".)