Curved shape different outside boundary (7)

I believe the answer is:

ellipse

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'curved shape' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

(Other definitions for ellipse that I've seen before include "Inclined plane through a cone" , "Figure like flattened circle" , "Sell pie badly to get a geometrical figure" , "Egg-shaped geometrical figure" , "Shape - sell pie (anag)" .)

I've seen this clue in The Sun.

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