Knocks down packs of cards (5)
I believe the answer is:
decks
This clue is a double definition.
'knocks down' is the first definition.
('deck' can be a synonym of 'knock down')
'packs of cards' is the second definition.
(I've seen this in another clue)
(Other definitions for decks that I've seen before include "Platforms on a vessel" , "Packs of playing cards" , "Packs of cards (US)" , "Embellishes" , "Floors" .)