Heard fish on the radio (5)
I believe the answer is:
tuner
'the radio' is the definition.
('tuner' can be a synonym of 'radio')
'heard fish' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'fish' becomes 'tuna' (tuna is a type of fish).
'tuna' sounds like 'TUNER'.
'on' acts as a link.
(Other definitions for tuner that I've seen before include "Adjuster of pianos" , "Piano adjuster" , "piano man?" , "Motor specialist" , "Radio receiver" .)