Track broadcast on the radio (5)
I believe the answer is:
scent
'track' is the definition.
(thesaurus)
'broadcast on the radio' is the wordplay.
'broadcast' becomes 'sent' (I've seen this in other clues).
'on the radio' indicates a 'sounds like' (homophone) clue.
'sent' is a homophone of 'SCENT'.
(Other definitions for scent that I've seen before include "Clue" , "Fragrance" , "Whiff" , "Distinctive smell" , "Animal smell" .)