Service vehicle pulled up on side of motorway (4)
I believe the answer is:
navy
'service' is the definition.
(navy is a kind of service)
'vehicle pulled up on side of motorway' is the wordplay.
'vehicle' becomes 'van' (type of vehicle).
'pulled up' says the letters should be written in reverse.
'on side of' says to take the final letters (one of the side letters is used).
The last letter of 'motorway' is 'y'.
'van' in reverse letter order is 'nav'.
'nav'+'y'='NAVY'
(Other definitions for navy that I've seen before include "Armed force" , "Shade of the marine forces" , "Blue - ships and seamen" , "Fleet - blue" , "Nation's warships" .)