He has no faith in Castro (7)
I believe the answer is:
infidel
'he has no faith' 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 rejecting a specific religion" , "Person who does not believe in a religion" , "Someone who does not acknowledge your god" , "Person not operating in good faith" , "Unbeliever" .)