Big wave from posh car (6)
I believe the answer is:
roller
I believe this is a double definition.
'big wave' is the first definition.
'posh car' is the second definition.
(abbreviation for Rolls Royce)
'from' acts as a link.
(Other definitions for roller that I've seen before include "Cylinder for flattening etc" , "Long heavy sea-wave" , "Cylinder for lawn etc" , "Wave - wave-maker" , "A large wave or a hair shaper" .)