Fourth dimension (4)
I believe the answer is:
time
'fourth dimension' is the definition.
(time is sometimes considered the fourth dimension)
This is all the clue.
(Other definitions for time that I've seen before include "Hours clocked up?" , "Continuum of past, present and future" , "The father of Truth, presumably" , "'. . . and tide wait for no man'" , "Duration; tempo" .)