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 anagramming the letters.
'tims' anagrammed gives 'MIST'.
'in' is the link.
(Other definitions for mist that I've seen before include "Fret" , "Watery film" , "Obscure" , "Thin layer of fog" , "Cloud; thin fog" .)