Arranged not as a musical work (6)
I believe the answer is:
sonata
'musical work' is the definition.
(I've seen this before)
'arranged not as a' is the wordplay.
'arranged' indicates anagramming the letters.
'not'+'as'='notas'
'notas' is an anagram of 'sonat'.
'sonat'+'a'='SONATA'
(Other definitions for sonata that I've seen before include "Piece of music for solo piano" , "Solo instrumental composition" , "Musical composition for a solo instrument" , "musical performance" , "just for one or two players?" .)