Zero common sense applied to motorway is threatening (7)
I believe the answer is:
ominous
'is threatening' is the definition.
('ominous' can be a synonym of 'threatening')
'zero common sense applied to motorway' is the wordplay.
'zero' becomes 'o'.
'common sense' becomes 'nous' (nous is a kind of common sense).
'applied to' says to put letters next to each other.
'motorway' becomes 'mi' (resembles M1, English motorway).
'nous' put after 'mi' is 'minous'.
'o'+'minous'='OMINOUS'
(Other definitions for ominous that I've seen before include "Threatening, not looking good" , "Sinister-looking" , "Inauspicious" , "Portending something bad" , "Foreboding, menacing" .)