Knock down part of building (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)
'part of building' is the third definition.
('floor' is part of 'building')
(Other definitions for floor that I've seen before include "Minimum (price) level" , "Knock down - planking" , "at bottom of forest" , "Dumbfound" , "Inside horizontal surface" .)