Flipping cheek, about to claim win! (7)
I believe the answer is:
prevail
'win' is the definition.
(I know that prevail is a more specific form of the action win)
'flipping cheek about to claim' is the wordplay.
'flipping' is a reversal indicator.
'cheek' becomes 'lip' (both can mean insolence or rudeness).
'about' indicates putting letters inside.
'to claim' becomes 'aver' (aver means to state or claim something).
'lip' enclosing 'aver' is 'liaverp'.
'liaverp' back-to-front is 'PREVAIL'.
(Other definitions for prevail that I've seen before include "come out on top" , "Exist generally" , "get the upper hand" , "Successfully persuade" , "Carry the day" .)