Stop luxury car hemming in a road-mending vehicle (11)
I believe the answer is:
steamroller
'road-mending vehicle' is the definition.
(I know that steamrollers are used to repair roads)
'stop luxury car hemming in a' is the wordplay.
'stop' becomes 'stem' (stem can mean to stop or interrupt).
'luxury car' becomes 'roller' (short for Rolls Royce).
'hemming in' means one lot of letters goes inside another (inserted letters are hemmed in).
'stem'+'roller'='stemroller'
'stemroller' going around 'a' is 'STEAMROLLER'.
(Other definitions for steamroller that I've seen before include "Forcibly crush" , "Knock flat" , "slow vehicle" , "Heavy vehicle" .)