Knocking code supremo? (7)
I believe the answer is:
pinking
'knocking' is the definition.
(both can mean hitting)
'code supremo?' is the wordplay.
'code' becomes 'pin' (a PIN number is a 4-digit code).
'supremo?' becomes 'king' (both can refer to a person with great authority).
'pin'+'king'='PINKING'
(Other definitions for pinking that I've seen before include "King pin (anag.)" , "(Engine) misfiring; type of shears" .)