Altering shape (8)

I believe the answer is:

triangle

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'altering shape' is the definition.
'triangle' can be an answer for 'shape' (triangle is a kind of polygon). I am not certain of the 'altering' bit.

'altering shape' is the wordplay.
'shape' indicates anagramming the letters.
'altering' anagrammed gives 'TRIANGLE'.

Can you help me to learn more?

(Other definitions for triangle that I've seen before include "Three-sided percussion instrument" , "Percussion instrument of geometric origin" , "Large tin (anag.)" , "diamond cut in half?" , "complex romantic situation?" .)

I've seen this clue in The Irish Independent.

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