Altering peculiar shape (8)

I believe the answer is:

triangle

'shape' is the definition.
(triangle is a kind of shape)

'altering peculiar' is the wordplay.
'peculiar' indicates anagramming the letters.
'altering' is an anagram of 'TRIANGLE'.

(Other definitions for triangle that I've seen before include "Polygon, could be isosceles" , "Simple percussion instrument" , "Three-sided percussion instrument" , "Relating to percussion instrument" , "Percussion instrument of geometric origin" .)

I've seen this clue in The Telegraph.

Want a hint initially instead of a full solution? Install my app