Regret referring to signalling code (7)
I believe the answer is:
remorse
'regret' is the definition.
(remorse is a kind of regret)
'referring to signalling code' is the wordplay.
'referring to' becomes 're'.
'signalling code' becomes 'morse' (Morse code).
're'+'morse'='REMORSE'
(Other definitions for remorse that I've seen before include "Feeling of guilt" , "Regret for wrong" , "Sorrow for wrong committed" , "Sorrow for sin or wrong-doing" , "Regret or sorrow for wrong-doing" .)