Triangle with two equal sides (9)
I believe the answer is:
isosceles
'triangle with two equal sides' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for isosceles that I've seen before include "Of triangular, with two sides of equal length" , "Kind of triangle" , "(Of triangles) having two sides the same" , "Triangle with two sides of equal length" , "Describing a triangle with two equal sides" .)