Excellent at deciphering codes (8)
I believe the answer is:
cracking
'excellent' is the definition.
(cracking can mean excellent or brilliant)
'at deciphering codes' is the wordplay.
I cannot quite understand how this works, but
'at' could be 'in' (synonymous in some cases - eg at school, in school) and 'in' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cracking that I've seen before include "failing" , "very fast" , "Excellent; fracturing" , "Solving" , "Wonderful" .)