Drink — twice it gets knocked over (4)
I believe the answer is:
asti
'drink' is the definition.
(I've seen this before)
'twice it gets knocked over' is the wordplay.
'twice' becomes 'as' (I can't justify this - if you can you should believe this answer much more).
'gets knocked over' shows that the letters should be reversed in order (I've seen 'getting knocked over' mean this).
'it' backwards is 'ti'.
'as'+'ti'='ASTI'
(Other definitions for asti that I've seen before include "It's a (anag) - wine" , "Italian town producing sparkling wine" , "Piedmont wine city, SE of Turin" , "Italian drunk" , "Italian 'sparkler'" .)