Sell pie - eccentric shape (7)
I believe the answer is:
ellipse
'shape' is the definition.
(ellipse is a kind of shape)
'sell pie eccentric' is the wordplay.
'eccentric' indicates anagramming the letters.
'sell'+'pie'='sellpie'
'sellpie' with letters rearranged gives 'ELLIPSE'.
(Other definitions for ellipse that I've seen before include "Regular oval shape" , "See pill (anag) - oval" , "Sell pie badly to get a geometrical figure" , "Egg-shaped geometrical figure" , "Geometrical oval" .)