Clearly let on the radio (5)
I believe the answer is:
aloud
'clearly' is the definition.
(thesaurus)
'let on the radio' is the wordplay.
'let' becomes 'allowed' (I've seen this in another clue).
'on the radio' indicates a 'sounds like' (homophone) clue.
'allowed' is a homophone of 'ALOUD'.
(Other definitions for aloud that I've seen before include "Take part in, link" , "that listeners can receive" , "Not sotto voce" , "At some volume" , "it is said" .)