Not equilateral or isosceles (7)
I believe the answer is:
scalene
'not equilateral or isosceles' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for scalene that I've seen before include "Having no sides for a match" , "(Of triangle) with no sides equal" , "not having evenly matched sides" , "(Of triangle) with each side of a different length" , "Describing a triangle with unequal sides" .)