Idiot crossing motorway? That’s out of order (5)
I believe the answer is:
amiss
'out of order' is the definition.
(synonyms)
'idiot crossing motorway?' is the wordplay.
'idiot' becomes 'ass' (both can mean a stupid person).
'crossing' is an insertion indicator (to cross can mean to go over or around).
'motorway?' becomes 'MI' (resembles M1, English motorway).
'ass' enclosing 'mi' is 'AMISS'.
'that's' acts as a link.
(Other definitions for amiss that I've seen before include "Mistakenly" , "Wrongly" , "Wrong - awry" , "Mistaken" , "Wrong; inaccurate" .)