Figure in Kepler's first law (7)
I believe the answer is:
ellipse
'figure in kepler's first law' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ellipse that I've seen before include "Sell pie off for a geometric figure" , "Sell pie badly to get a geometrical figure" , "See pill (anag) - oval" , "Inclined plane through a cone" , "Egg-shaped geometrical figure" .)