Radio talk is about a Greek character (4)
I believe the answer is:
iota
'a greek character' is the definition.
(letter from the Greek alphabet)
'radio talk is about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'IOTA' can be found hidden inside 'radio talk is'.
(Other definitions for iota that I've seen before include "The smallest possible amount, or the Greek i" , "Greek letter; tiny amount" , "Just a jot, from Greek alphabet" , "Jot or tittle" , "Greek equivalent of the letter I" .)