Boy gets a thank you for composition (6)
I believe the answer is:
sonata
'for composition' is the definition.
(type of musical composition)
'boy gets a thank you' is the wordplay.
'boy' becomes 'son' (synonyms).
'gets' is a charade indicator (letters next to each other).
'thank you' becomes 'ta' (informal way of saying thank you).
'son'+'a'+'ta'='SONATA'
(Other definitions for sonata that I've seen before include "Eg, Beethoven's Kreutzer or Spring" , "Music for a soloist" , "Musical work for a solo instrument" , "Moonlight, perhaps" , "A musical form" .)