Motorway exit (8)
(Other definitions for slip road that I've seen before include "way to access motorway" , "Motorway approach or exit" , "Motorway exit or entrance" , "Joining lane" , "Way to leave or join a motorway" .)
'motorway exit' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for junction that I've seen before include "See 7" , "Place where railway lines meet and join" , "Intersection" , "Point where ways meet" , "Union - joint" .)