Steamroller (9)
I believe the answer is:
overwhelm
'steamroller' is the definition.
(I know that steamroller is a more specific form of overwhelm)
This is the entire clue.
(Other definitions for overwhelm that I've seen before include "Swamp - have a strong emotional effect on" , "Inundate, engulf" , "Completely defeat" , "Bury or drown beneath a huge mass of something" , "Vanquish" .)