Regret concerning code (7)
I believe the answer is:
remorse
'regret' is the definition.
(remorse is a feeling of regret)
'concerning code' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'code' becomes 'morse' (Morse code).
're'+'morse'='REMORSE'
(Other definitions for remorse that I've seen before include "Sorrow for wrong committed" , "Regret or sorrow for wrong-doing" , "Feeling of regret for wrongdoing" , "Contrition for wrongdoing" , "Guilt for a wrong deed" .)