Speed restriction on an externally lit road (5)
I believe the answer is:
limit
'speed restriction' is the definition.
'limit' can be an answer for 'restriction' (I've seen this before). I am not sure about the 'speed' bit.
'on an externally lit road' is the wordplay.
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'an' becomes 'i' (Roman numeral for one).
'externally' means one lot of letters goes inside another (some letters go on the outside of others).
'road' becomes 'M'.
'lit' placed around 'm' is 'limt'.
'i' placed within 'limt' is 'LIMIT'.
Can you help me to learn more?
(Other definitions for limit that I've seen before include "qualify?" , "Confine within bounds" , "Restrict - term" , "Ration" , "Breaking point" .)