Crossword Genius

Altering incorrect figure (8)

Ross

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" .)

I've seen this clue in The Telegraph.
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