One who doesn't believe in Castro? (7)
I believe the answer is:
infidel
'one who doesn't believe' is the definition.
(term for someone with no religion)
'in castro?' is the wordplay.
'castro?' becomes 'fidel' (Fidel Castro famous ruler of Cuba).
'in'+'fidel'='INFIDEL'
(Other definitions for infidel that I've seen before include "Heathen" , "Non-believer, heretic" , "Pagan" , "One who doesn't believe" , "Person who rejects a religious belief" .)