Altering incorrect figure (8)
I believe the answer is:
triangle
'figure' is the definition.
(triangle is a kind of figure)
'altering incorrect' is the wordplay.
'incorrect' indicates an anagram.
'altering' anagrammed gives 'TRIANGLE'.
(Other definitions for triangle that I've seen before include "Three-sided figure (maths)" , "Three-sided polygon" , "diamond cut in half?" , "Integral part of common geometric figure" , "Relating to percussion instrument" .)