One who’s succeeded in broadcast once before (3)
I believe the answer is:
ere
'once before' is the definition.
(archaic term for 'before')
'one who's succeeded in broadcast' is the wordplay.
'one who's succeeded' becomes 'heir' (an heir is someone's successor).
'in broadcast' indicates a 'sounds like' (homophone) clue.
'heir' is a homophone of 'ERE'.
(Other definitions for ere that I've seen before include "Before (poetic)" , "Before (in poetry)" , "Archaic word for before - a palindrome" , "Old prior" , "One who steals" .)