Boy attending to a composition (6)
I believe the answer is:
sonata
'composition' is the definition.
(thesaurus)
'boy attending to a' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'attending' becomes 'at' (I've seen this in other clues).
'son'+'at'+'a'='SONATA'
(Other definitions for sonata that I've seen before include "See 6" , "Eg, Beethoven's Kreutzer or Spring" , "musical performance" , "Instrumental composition for soloist" , "Solo piano composition" .)