He doesn't believe in Castro (7)
I believe the answer is:
infidel
'he doesn't believe' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'in castro' is the wordplay.
'castro' becomes 'fidel' (Fidel Castro famous ruler of Cuba).
'in'+'fidel'='INFIDEL'
Can you help me to learn more?
(Other definitions for infidel that I've seen before include "Person who does not believe in a religion" , "Person who has no faith, or the wrong one" , "One without religion (mainly archaic)" , "Person who rejects a religious belief" , "Person not operating in good faith" .)