Passage on the radio could be key (4)
I believe the answer is:
isle
'be key' is the definition.
(key can mean a small island)
'passage on the radio' is the wordplay.
'passage' becomes 'aisle' (aisle is a kind of passage).
'on the radio' shows a homophone (sound like).
'aisle' sounds like 'ISLE'.
'could' acts as a link.
(Other definitions for isle that I've seen before include "The Emerald - (Ireland)" , "Man, possibly" , "Man, maybe" , "Eg, Staffa" , "Atoll" .)