Knock out with weight — ouch! Fall on the floor? (6)
I believe the answer is:
kowtow
'fall on the floor?' is the definition.
(kowtow can mean to kneel in Chinese custom)
'knock out with weight ouch' is the wordplay.
'knock out' becomes 'ko'.
'with' is a charade indicator (letters next to each other).
'weight' becomes 'wt'.
'ouch' becomes 'ow' (interjection synonym).
'ko'+'wt'+'ow'='KOWTOW'
(Other definitions for kowtow that I've seen before include "Behave obsequiously" , "Abase oneself" , "Make oriental sort of obeisance" , "Make a deferential gesture" , "Gesture of submission" .)