Checker, perhaps, who won't be sticking at twenty-one (7)
I believe the answer is:
twister
This clue is a double definition.
'checker perhaps' is the first definition.
(Chubby Checker recorded the song The Twist)
'who won't be sticking at twenty-one' is the second definition.
(as in sticking or twisting in the card game 21)
(Other definitions for twister that I've seen before include "Tornado (colloq.)" , "Funnel-shaped wind" , "Crook - tornado" , "One dancing" , "A swindler or a cyclone" .)