Motorway madness an illusion? (6)
I believe the answer is:
mirage
'illusion?' is the definition.
(I've seen this before)
'motorway madness' is the wordplay.
'motorway' becomes 'MI' (resembles M1, English motorway).
'madness' becomes 'rage' (synonyms).
'mi'+'rage'='MIRAGE'
'an' acts as a link.
(Other definitions for mirage that I've seen before include "Deceptive image, in the desert say" , "It doesn't exist" , "Something illusory and unattainable" , "Illusory image" , "You might see this in desert" .)