Perfidious, but his gate is famous (7)
I believe the answer is:
traitor
'perfidious but his' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'gate is famous' is the wordplay.
I cannot quite see how this works, but
'gate' could be 'or' (or gate is a kind of gate) and 'or' is found within the answer.
The remaining letters 'trait' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for traitor that I've seen before include "Judas" , "Betrayer, quisling" , "Disloyal person" , "defector?" , "Not a good subject" .)