Crossword Genius

Not seen, heard in fog (4)

Ross

I believe the answer is:

mist

'fog' is the definition.
(I know that fog can be written as mist)

I don't understand the rest of the clue.

(Other definitions for mist that I've seen before include "Watery vapour" , "Cloud; thin fog" , "Think fog or low cloud" , "low-level cloud" , "Slight fog" .)

I've seen this clue in The Sun.
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