I triumphed in broadcast (3)
I believe the answer is:
one
'i' is the definition.
(Roman numeral for one)
'triumphed in broadcast' is the wordplay.
'triumphed' becomes 'won' (I've seen this before).
'in broadcast' indicates a 'sounds like' (homophone) clue.
'won' sounds like 'ONE'.
(Other definitions for one that I've seen before include "This is singular" , "Single and undivided" , "Smallest whole number" , "SECOND PART OF 16" , "Lowest cardinal number" .)