Unbeliever at home with Castro (7)
I believe the answer is:
infidel
'unbeliever' is the definition.
(I've seen this before)
'home with castro' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'with' is a charade indicator (letters next to each other).
'castro' becomes 'fidel' (Fidel Castro famous ruler of Cuba).
'in'+'fidel'='INFIDEL'
'at' acts as a link.
(Other definitions for infidel that I've seen before include "I don't believe" , "Literally, 9 [UNFAITHFUL]" , "Person rejecting a specific religion" , "Heathen" , "One who doesn't believe" .)