Crossword Genius

Excellent at deciphering codes (8)

Ross

I believe the answer is:

cracking

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Evening Standard.
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