Imposter has cycled on motorway (4)
I believe the answer is:
sham
'imposter' is the definition.
(I know that imposter can be written as sham)
'has cycled on motorway' is the wordplay.
'cycled' indicates anagramming the letters.
'on' says to put letters next to each other.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'has' anagrammed gives 'sha'.
'sha'+'m'='SHAM'
(Other definitions for sham that I've seen before include "Phoney kind of hams" , "Mash, being counterfeit" , "Pretence; bogus, false" , "Bogus hams" , "Unreal" .)