Radio announcement’s to recognise a worry: coastal fog (3,4)
I believe the answer is:
Here is my best explanation:
'coastal fog' is the definition.
'radio announcement's to recognise a worry' is the subsidiary indication.
'radio announcement's' indicates a 'sounds like' (homophone) clue.
'to recognise' becomes 'see'.
'a worry' becomes 'fret' (I've seen this before).
'see' sounds like 'sea'.
(Other definitions for sea fret that I've seen before include "Coastal mist", "North Sea haar, for instance" and "Mist coming inland at the coast".)