Tim's lost in obscurity (4)
I believe the answer is:
mist
'obscurity' is the definition.
(thesaurus)
'tim's lost' is the wordplay.
'lost' indicates an anagram.
'tims' is an anagram of 'MIST'.
'in' acts as a link.
(Other definitions for mist that I've seen before include "Fret" , "Watery film" , "Obscure" , "Thin layer of fog" , "Cloud; thin fog" .)