Tim's lost in lack of visibility (4)
I believe the answer is:
mist
'lack of visibility' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'tim's lost' is the wordplay.
'lost' indicates anagramming the letters.
'tims' with letters rearranged gives 'MIST'.
'in' is the link.
Can you help me to learn more?
(Other definitions for mist that I've seen before include "Fine spray" , "Fret" , "Watery film" , "Cloud; thin fog" , "Obscure" .)