Beginning to signal in additional code (5)
I believe the answer is:
morse
'code' is the definition.
(morse code is a kind of code)
'beginning to signal in additional' is the wordplay.
'beginning to' indicates taking the first letters.
'in' is an insertion indicator.
'additional' becomes 'more' ('more' can be a synonym of 'additional').
The first letter of 'signal' is 's'.
's' placed inside 'more' is 'MORSE'.
(Other definitions for morse that I've seen before include "& 24 Across Encrypted language" , "used to transmit messages" , "Samuel ---, telegraph inventor" , "Code inventor, Samuel" , "Walrus - TV detective" .)