Not as jazzy a composition (6)
I believe the answer is:
sonata
'composition' is the definition.
(thesaurus)
'not as jazzy a' is the wordplay.
'jazzy' indicates an anagram.
'not'+'as'='notas'
'notas' with letters rearranged gives 'sonat'.
'sonat'+'a'='SONATA'
(Other definitions for sonata that I've seen before include "just for one or two players?" , "Instrumental composition for soloist" , "Beethoven's 'Moonlight', for example" , "Solo piano composition" , "Musical piece" .)