Cheek, if not round, then oval (7)
I believe the answer is:
ellipse
'oval' is the definition.
(I know that oval can be written as ellipse)
'cheek if not round' is the wordplay.
'cheek' becomes 'lip' (both can mean insolence or rudeness).
'if not' becomes 'else' (synonyms).
'round' is an insertion indicator.
'lip' going inside 'else' is 'ELLIPSE'.
'then' acts as a link.
(Other definitions for ellipse that I've seen before include "Figure like flattened circle" , "Inclined plane through a cone" , "Shape - sell pie (anag)" , "Geometrical oval" , "Sell pie off for a geometric figure" .)