Item moving in fourth dimension (4)
I believe the answer is:
time
'fourth dimension' is the definition.
(time is sometimes considered the fourth dimension)
'item moving' is the wordplay.
'moving' indicates an anagram.
'item' is an anagram of 'TIME'.
'in' acts as a link.
(Other definitions for time that I've seen before include "Measure of duration" , "Rhythm" , "weekly magazine" , "> moment" , ""The enemy"" .)