Crossword Genius

Percussion instrument, altering when beaten (8)

Ross

I believe the answer is:

triangle

'percussion instrument' is the definition.
(triangle is a kind of percussion instrument)

'altering when beaten' is the wordplay.
'when beaten' indicates anagramming the letters (letters beaten into a new form).
'altering' with letters rearranged gives 'TRIANGLE'.

(Other definitions for triangle that I've seen before include "Integral (anag) - geometrical shape", "It could be eternal", "It has three sides", "Geometric shape (eternal, maybe)", "Three-sided percussion instrument".)

I've seen this clue in The Guardian.