Talk Radio is holding back secret (4)
I believe the answer is:
dark
'secret' is the definition.
(thesaurus)
'talk radio is holding back' is the wordplay.
'is holding' says the answer is hidden in the clue.
'back' says the letters should be written backwards.
'krad' is hidden within 'talk radio'.
'krad' written backwards gives 'DARK'.
(Other definitions for dark that I've seen before include "Black, perhaps - night" , "Sinister - evening" , "Sombre in colour" , "Overcast" , "Such an unknown horse" .)