Item lost in fourth dimension (4)
I believe the answer is:
time
'fourth dimension' is the definition.
(time is sometimes considered the fourth dimension)
'item lost' is the wordplay.
'lost' is an anagram indicator.
'item' is an anagram of 'TIME'.
'in' is the link.
(Other definitions for time that I've seen before include "> moment" , "Finite duration" , "Rhythm" , "The father of Truth, presumably" , "Measure of duration" .)