Main character on the radio (3)
I believe the answer is:
sea
'main' is the definition.
(the main is an old name for the sea)
'character on the radio' is the wordplay.
'character' becomes 'see' (possible spelling of 'C' according to Chambers).
'on the radio' shows a homophone (sound like).
'see' is a homophone of 'SEA'.
(Other definitions for sea that I've seen before include "70% of the earth's surface" , "Marine" , "The Med, e.g." , "E.g Aral or Weddell" , "Part of ocean" .)