Working on radio is a beginning (5)
I believe the answer is:
onset
'a beginning' is the definition.
(onset is a kind of beginning)
'working on radio' is the wordplay.
'working' becomes 'on'.
'on' is a charade indicator (letters next to each other).
'radio' becomes 'set' (I've seen this in other clues).
'on'+'set'='ONSET'
'is' acts as a link.
(Other definitions for onset that I've seen before include "Assault; start" , "Start - notes (anag)" , "Start, beginning" , "First signs" , "Start - opening moments" .)