Link across motorway - to the fourth dimension! (4)
I believe the answer is:
time
'the fourth dimension' is the definition.
('time' can be a synonym of 'fourth dimension')
'link across motorway' is the wordplay.
'link' becomes 'tie' ('tie' can be a synonym of 'link'**).
'across' indicates putting letters inside.
'motorway' becomes 'm'.
'tie' going around 'm' is 'TIME'.
'to' acts as a link.
(Other definitions for time that I've seen before include "'. . . and tide wait for no man'" , "Prison sentence (colloq.)" , "Pub closing call" , "Speech to close pub" , "Era" .)