Thin fog may be avoided, we hear (4)
I believe the answer is:
mist
'thin fog' is the definition.
(I've seen this before)
'avoided we hear' is the wordplay.
'avoided' becomes 'missed' (missing is a kind of avoiding).
'we hear' shows a homophone (sound like).
'missed' is a homophone of 'MIST'.
'may be' is the link.
(Other definitions for mist that I've seen before include "Obscure" , "Watery vapour" , "Slight fog" , "Fine spray" , "Cloud; thin fog" .)