Certain coastal area is on the radio (4)
I believe the answer is:
sure
'certain' is the definition.
(I know that certain can be written as sure)
'coastal area is on the radio' is the wordplay.
'coastal area' becomes 'shore' (I've seen this before).
'is on the radio' indicates a 'sounds like' (homophone) clue.
'shore' is a homophone of 'SURE'.
(Other definitions for sure that I've seen before include "Definite" , "Positive" , "certainly in the USA" , "Certain to be a ruse" , "The old sewer" .)