Knock bust over (4)
I believe the answer is:
stub
'knock' is the definition.
'bust over' is the wordplay.
'over' indicates anagramming the letters.
'bust' anagrammed gives 'STUB'.
(Other definitions for stub that I've seen before include "Pencil/cigarette remnant" , "Strike against hard object - one's toe, perhaps" , "what smoker drops in street?" , "Residue of cigarette or cheque" , "Broken-off end" .)