Boy volunteers to pen a piece of music (6)
I believe the answer is:
sonata
'piece of music' is the definition.
(I've seen this before)
'boy volunteers to pen a' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'to pen' indicates putting letters inside.
'son'+'ta'='sonta'
'sonta' placed around 'a' is 'SONATA'.
(Other definitions for sonata that I've seen before include "just for one or two players?" , "Work for soloist" , "Classical composition" , "Solo instrumental composition" , "Musical composition for a solo instrument" .)