Instruments in largest composition (9)
I believe the answer is:
triangles
'instruments' is the definition.
(triangle is a kind of instrument)
'in largest composition' is the wordplay.
'composition' is an anagram indicator (letters arranged or composed in a different order).
'in'+'largest'='inlargest'
'inlargest' anagrammed gives 'TRIANGLES'.
(Other definitions for triangles that I've seen before include "Musical instruments" , "Three-sided polygons" , "Three-sided shapes" , "Percussion instruments" , "Three-sided figures" .)