Big road filled by trendy car (4)
I believe the answer is:
mini
'car' is the definition.
(Mini Cooper)
'big road filled by trendy' is the wordplay.
'big road' becomes 'mi' (resembles M1, English motorway).
'filled by' means one lot of letters goes inside another.
'trendy' becomes 'in' ('in' can mean fashionable).
'mi' placed around 'in' is 'MINI'.
(Other definitions for mini that I've seen before include "Brief" , "Baby" , "Italian Job car?" , "(Car etc) of small size" , "Iconic economy car (introduced in 1959)" .)