Code-breaker, person of exceptional ability (7)
I believe the answer is:
cracker
This clue is a double definition.
'code-breaker' is the first definition.
(someone cracking a code)
'person of exceptional ability' is the second definition.
(cracker can mean someone extremely skilled)
(Other definitions for cracker that I've seen before include "Noisy firework" , "Biscuit - banger - brilliant one" , "Thin biscuit or loud party favourite" , "Excellent" , "See 8" .)