Allow a day - the motorway traffic starts (5)
I believe the answer is:
admit
'allow' is the definition.
(admitting is a kind of allowing)
'a day the motorway traffic starts' is the wordplay.
'a day' becomes 'ad' (I've seen this in other clues**).
'the motorway' becomes 'mi' (resembles Ma, English motorway).
'starts' says to take the initial letters.
The first letter of 'traffic' is 't'.
'ad'+'mi'+'t'='ADMIT'
(Other definitions for admit that I've seen before include "Come clean" , "Stop being obstructive" , "receive" , "Usher in" , "Accept" .)