Aficionado playing it as a musical composition (8)
I believe the answer is:
fantasia
'musical composition' is the definition.
(I know that fantasia is a type of musical composition)
'aficionado playing it as a' is the wordplay.
'aficionado' becomes 'fan' (I've seen this before).
'playing' indicates an anagram.
'it'+'as'='itas'
'itas' with letters rearranged gives 'tasi'.
'fan'+'tasi'+'a'='FANTASIA'
(Other definitions for fantasia that I've seen before include "Disney film of cartoons on classical music" , "piece of music" , "Musical composition in an improvised style" , "potpourri of tunes" , "Improvisatory musical piece" .)