Fool -- crossing motorway must be wrong! (5)
I believe the answer is:
amiss
'wrong' is the definition.
('amiss' can be a synonym of 'wrong')
'fool crossing motorway' is the wordplay.
'fool' becomes 'ass' (ass is a kind of fool**).
'crossing' indicates putting letters inside.
'motorway' becomes 'mi' (resembles Ma, English motorway).
'ass' enclosing 'mi' is 'AMISS'.
'must be' is the link.
(Other definitions for amiss that I've seen before include "Badly - wrong" , "error" , "(Gone) wrong" , "Not quite right" , "Mistakenly" .)