One dimension (4)
I believe the answer is:
time
'one dimension' is the definition.
(time is sometimes considered the fourth dimension)
This is the entire clue.
(Other definitions for time that I've seen before include "weekly magazine" , "Prison sentence (colloq.)" , "'. . . and tide wait for no man'" , "Indefinite period" , "Continuum of past, present and future" .)