Motorway madness? It’s just imaginary! (6)
I believe the answer is:
mirage
'just imaginary' is the definition.
I can't tell whether this definition defines the answer.
'motorway madness?' is the wordplay.
'motorway' becomes 'mi' (resembles M1, English motorway).
'madness?' becomes 'rage' (synonyms).
'mi'+'rage'='MIRAGE'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for mirage that I've seen before include "It doesn't exist" , "Deceptive image, in the desert say" , "French flier" , "Illusion caused by atmospheric refraction" , "French fighter aircraft" .)