Percussion instrument, altering when beaten (8)
'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".)