Knocks down storeys (6)
I believe the answer is:
floors
I believe this clue is a double definition.
'knocks down' is the first definition.
('floor' can be a synonym of 'knock down')
'storeys' is the second definition.
(I know this)
(Other definitions for floors that I've seen before include "Levels of a building" , "Knocks down with a punch" , "Knocks person down in a fight or confounds" , "Baffles completely" , "Inside horizontal surfaces" .)