Island broadcaster on air (4)
I believe the answer is:
skye
'island' is the definition.
(Scottish island)
'broadcaster on air' is the wordplay.
'broadcaster' becomes 'sky' (Sky is a television company).
'on air' indicates a 'sounds like' (homophone) clue (how the word sounds on the radio, on air).
'sky' sounds like 'SKYE'.
(Other definitions for skye that I've seen before include "Largest island of Scotland's Inner Hebrides" , "Scot. island, toll bridge" , "Largest isle of the Inner Hebrides" , "Portree island" , "Scottish isle" .)