Knocking over (9)
I believe the answer is:
upsetting
'knocking over' is the definition.
('upset' can be a synonym of 'knock over')
This is all the clue.
(Other definitions for upsetting that I've seen before include "sad?" , "Disturbing or upturning" , "Likely to put out" , "Alarming" , "Tipping" .)