Not seen, heard in fog (4)
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" .)