Composer non-believer? In one! (8)
I believe the answer is:
paganini
'composer' is the definition.
(I've seen this before)
'non-believer? in one' is the wordplay.
'non-believer?' becomes 'pagan' (I've seen this in another clue).
'one' becomes 'i' (Roman numeral).
'pagan'+'in'+'i'='PAGANINI'
(Other definitions for paganini that I've seen before include "Famous Italian violinist" , "Violin virtuoso and composer" , "Famous violin virtuoso" , "Niccolo _; Italian composer" , "One on the fiddle" .)