Restriction illuminated across motorway (5)
I believe the answer is:
limit
'restriction' is the definition.
(I've seen this in another clue)
'illuminated across motorway' is the wordplay.
'illuminated' becomes 'lit' (synonyms).
'across' is an insertion indicator.
'motorway' becomes 'MI' (resembles M1, English motorway).
'lit' enclosing 'mi' is 'LIMIT'.
(Other definitions for limit that I've seen before include "Constrain" , "Bound, restriction" , "Restrict - boundary" , "Constraint" , "Ration" .)