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 "Mist coming inland at the coast", "phenomenon coming inland?", "North Sea haar, for instance", "onshore mist", "Fog", "Threat to visibility", "coastal haze" and "Coastal mist".)