Altering puzzle's form (8)

I believe the answer is:

triangle

'form' is the definition.
(I know that triangle is a type of form)

'altering puzzle's' is the wordplay.
'puzzle's' is an anagram indicator (I've seen 'puzzles' mean this).
'altering' anagrammed gives 'TRIANGLE'.

(Other definitions for triangle that I've seen before include "kitchen item" , "Three-sided polygon" , "Percussion instrument of geometric origin" , "Geometric shape (eternal, maybe)" , "Gable shape -- 1 across (anag)" .)

I've seen this clue in the Sydney Morning Herald.

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