Knock the bust over (4)
I believe the answer is:
stub
'knock' is the definition.
'the bust over' is the wordplay.
'over' indicates anagramming the letters.
'bust' anagrammed gives 'STUB'.
(Other definitions for stub that I've seen before include "Extinguish the cigarette on a counterfoil" , "Tooth remnant" , "Short piece left over" , "evidence of smoking, for example" , "Wee bit of pencil, eg," .)