Enthusiast played it as a free musical composition (8)
I believe the answer is:
fantasia
'free musical composition' is the definition.
(I've seen this before)
'enthusiast played it as a' is the wordplay.
'enthusiast' becomes 'fan' (fan is a kind of enthusiast).
'played' is an anagram indicator.
'it'+'as'='itas'
'itas' is an anagram of 'tasi'.
'fan'+'tasi'+'a'='FANTASIA'
(Other definitions for fantasia that I've seen before include "Free musical composition" , "Disney film (1940)" , "Improvisatory composition" , "Improvisatory musical piece" , "Musical composition in an improvised style" .)