Key passage on the radio (4)
I believe the answer is:
isle
'key' is the definition.
(key can mean a small island)
'passage on the radio' is the wordplay.
'passage' becomes 'aisle' (aisle can mean a path or passageway).
'on the radio' shows a homophone (sound like).
'aisle' sounds like 'ISLE'.
(Other definitions for isle that I've seen before include "eg Hoy, Yell or Sark" , "-- of Ely, Dogs, Man" , "See 11 Across" , "Could be Man" , "Prince Edward perhaps" .)