Not found in poor visibility, we’re told (6)
I believe the answer is:
missed
'not found' is the definition.
(miss can mean to fail to find something)
'poor visibility we're told' is the wordplay.
'poor visibility' becomes 'mist' (mist causes poor visibility).
'we're told' indicates a 'sounds like' (homophone) clue.
'mist' sounds like 'MISSED'.
'in' is the link.
(Other definitions for missed that I've seen before include "Didn't catch" , "came too late" , "Overlooked; needed" , "Didn't attend" , "Failed to perceive" .)