Like a dunce cap (5)
I believe the answer is:
conic
'like a dunce cap' is the definition.
(dunce caps are conical)
This is all the clue.
(Other definitions for conic that I've seen before include "A 99 is thus" , "elliptical shape" , "Shaped like an ice cream cornet" , "solid, curved" , "Figure of circle, ellipse, parabola or hyperbola" .)