Light fog, by the sound of it, is left out (6)
I believe the answer is:
missed
'left out' is the definition.
('miss' can be a synonym of 'leave out')
'light fog by the sound of it' is the wordplay.
'light fog' becomes 'mist' (I've seen this in another clue).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'mist' sounds like 'MISSED'.
'is' is the link.
(Other definitions for missed that I've seen before include "Failed to perceive" , "failed to see" , "Didn't attend" , "Overlooked; needed" , "Avoided" .)