Priest protects Nigerian leader with gun (6)
I believe the answer is:
cannon
'gun' is the definition.
(cannon is a kind of gun)
'priest protects nigerian leader' is the wordplay.
'priest' becomes 'canon' (canon is a kind of priest).
'protects' means one lot of letters goes inside another.
'nigerian leader' becomes 'n' (I am not sure about this - if you are sure you should believe this answer much more).
'canon' enclosing 'n' is 'CANNON'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for cannon that I've seen before include "Gun - billiards' shot" , "Large gun that fires heavy shells" , "shot playing billiards" , "military weapon" , "Large gun mounted on wheels" .)