Disney film buff cheers one in South Africa (8)
I believe the answer is:
fantasia
'disney film' is the definition.
(1940 film)
'buff cheers one in south africa' is the wordplay.
'buff' becomes 'fan' (eg a film buff is a film fan).
'cheers' becomes 'ta' (both can informally mean thank you).
'one' becomes 'I' (Roman numeral).
'in' means one lot of letters goes inside another.
'south africa' becomes 'SA' (abbreviation).
'i' going inside 'sa' is 'sia'.
'fan'+'ta'+'sia'='FANTASIA'
(Other definitions for fantasia that I've seen before include "Fat as Ian (anag.)" , "Disney film of cartoons on classical music" , "Musical composition of several familiar themes" , "Musical composition in an improvised style" , "Improvisatory musical piece" .)