Motorway madness? It's an illusion (6)
I believe the answer is:
mirage
'an illusion' is the definition.
(thesaurus)
'motorway madness?' is the wordplay.
'motorway' becomes 'mi' (resembles M1, English motorway).
'madness?' becomes 'rage' ('rage' can be a synonym of 'madness').
'mi'+'rage'='MIRAGE'
'it's' acts as a link.
(Other definitions for mirage that I've seen before include "Hallucination caused by hot air" , "See it to believe it?" , "image, not what it seems" , "French fighter aircraft" , "fighter craft" .)