Island wharf radioed (3)
I believe the answer is:
key
'island' is the definition.
(key can mean an island or reef)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for key that I've seen before include "Vital" , "Lock opener; low island" , "It can open" , "Crucially important" , "Crucial arch-stone" .)