Son entering extra code name (5)
I believe the answer is:
morse
'code name' is the definition.
(Morse is the name of a code - Morse code)
'son entering extra' is the wordplay.
'son' becomes 's'.
'entering' is an insertion indicator.
'extra' becomes 'more' (I've seen this before).
's' placed within 'more' is 'MORSE'.
(Other definitions for morse that I've seen before include "Fictional inspector; code" , "Code; TV detective" , "Endeavour" , "Dot-dash code inventor" , "Oxford detective" .)