Boy volunteers clutching a composition (6)
I believe the answer is:
sonata
'composition' is the definition.
(type of musical composition)
'boy volunteers clutching a' is the wordplay.
'boy' becomes 'son' (synonyms).
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'clutching' indicates putting letters inside.
'son'+'ta'='sonta'
'sonta' enclosing 'a' is 'SONATA'.
(Other definitions for sonata that I've seen before include "composition for piano, perhaps" , "Piece of classical music" , "Eg, Beethoven's Kreutzer or Spring" , "Musical composition for a solo instrument" , "Solo piano composition" .)