Boy volunteers with a composition (6)
I believe the answer is:
sonata
'composition' is the definition.
(thesaurus)
'boy volunteers with a' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'with' means one lot of letters go next to another.
'ta' put after 'a' is 'ata'.
'son'+'ata'='SONATA'
(Other definitions for sonata that I've seen before include "Work for soloist" , "Moonlight, perhaps" , "Piece of music for solo piano" , "Solo instrumental composition" , "A musical form" .)