Fury on motorway? It's an illusion (6)
I believe the answer is:
mirage
'illusion' is the definition.
(I've seen this before)
'fury on motorway? it's an' is the wordplay.
'fury' becomes 'rage' (synonyms).
'on' means one lot of letters go next to another.
'motorway?' becomes 'M' (as in motorway names - Mr, Me etc).
'it's an' becomes 'i' (Roman numeral for one).
'm'+'i'='mi'
'rage' after 'mi' is 'MIRAGE'.
(Other definitions for mirage that I've seen before include "French flier" , "French fighter aircraft" , "Illusory image" , "Optical illusion caused by atmospheric conditions" , "image, not what it seems" .)