Knock down a storey (5)
I believe the answer is:
floor
I believe this clue is a triple definition!
'knock' is the first definition.
(thesaurus)
'down' is the second definition.
(thesaurus)
'a storey' is the third definition.
(I know that storey can be written as floor)
(Other definitions for floor that I've seen before include "Ground" , "Puzzle" , "American's story" , "Bottom surface" , "Stump" .)