Oliver, with some hesitation, becoming a crook (7)
I believe the answer is:
twister
'a crook' is the definition.
(I've seen this before)
'oliver with some hesitation' is the wordplay.
'oliver' becomes 'twist' (as in Oliver Twist).
'with' means one lot of letters go next to another.
'some hesitation' becomes 'er' (sound made while hesitating - um, er).
'twist'+'er'='TWISTER'
'becoming' is the link.
(Other definitions for twister that I've seen before include "conman" , "Swindler; tornado (US)" , "Colloquial name for a tornado" , "Crook - tornado" , "A swindler or a cyclone" .)