Completely defeat (9)
I believe the answer is:
overwhelm
'completely defeat' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for overwhelm that I've seen before include "Bury or drown beneath a huge mass of something" , "Swamp - have a strong emotional effect on" , "Too much" , "Crush" , "Overcome by superior force" .)