Item moving in the fourth dimension (4)
I believe the answer is:
time
'the fourth dimension' is the definition.
(time is sometimes considered the fourth dimension)
'item moving' is the wordplay.
'moving' is an anagram indicator (letters are moved around).
'item' anagrammed gives 'TIME'.
'in' is the link.
(Other definitions for time that I've seen before include "Hours clocked up?" , "See 15" , "Its detonation is delayed" , "Tempo; occasion" , "The fourth dimension?" .)