Worn on the foot, it can knock you down! (4)
I believe the answer is:
sock
This might be a double definition.
'worn on the foot it' is the first definition.
I know that 'foot' relates to this answer.
'knock you down' is the second definition.
'sock' can be an answer for 'knock' (thesaurus). I am not sure about the 'you down' bit.
'can' acts as a link.
Can you help me to learn more?
(Other definitions for sock that I've seen before include "Foot-covering" , "One worn" , "Wallop" , "Item of footwear" , "bash" .)