True sound on the radio (8)
I believe the answer is:
straight
'true' is the definition.
(synonyms)
'sound on the radio' is the wordplay.
'sound' becomes 'strait' (strait is a kind of sound).
'on the radio' indicates a 'sounds like' (homophone) clue.
'strait' is a homophone of 'STRAIGHT'.
(Other definitions for straight that I've seen before include "Honest, not crooked" , "Sat right in unbending way" , "in a row" , "Linear" , "Uncurved" .)