Motorway surrounded by illuminated hint of speed restrictions (6)
I believe the answer is:
limits
'restrictions' is the definition.
(I've seen this in another clue)
'motorway surrounded by illuminated hint of speed' is the wordplay.
'motorway' becomes 'MI' (resembles M1, English motorway).
'surrounded by' indicates putting letters inside.
'illuminated' becomes 'lit' (synonyms).
'hint of' indicates taking the first letters (the smallest possible amount of the word).
The first letter of 'speed' is 's'.
'mi' inserted inside 'lit' is 'limit'.
'limit'+'s'='LIMITS'
(Other definitions for limits that I've seen before include "Frontiers" , "Boundaries or restrictions" , "Points beyond which one may not go" , "Boundaries, confines" , "Biblical prophet" .)