Knock down storey (5)
I believe the answer is:
floor
This clue is a triple definition!
'knock' is the first definition.
(thesaurus)
'down' is the second definition.
(thesaurus)
'storey' is the third definition.
(I know that storey can be written as floor)
(Other definitions for floor that I've seen before include "Puzzle" , "Storey of a building" , "Bottom surface" , "Minimum (price) level" , "Stump" .)