Fool knocking container over (3)
I believe the answer is:
nit
'fool knocking' is the definition.
'nit' can be an answer for 'fool' (both can mean a stupid person). I'm not sure about the 'knocking' bit.
'container over' is the wordplay.
'container' becomes 'tin' (tin is a kind of container).
'over' says the letters should be written in reverse.
'tin' written backwards gives 'NIT'.
(Other definitions for nit that I've seen before include "Louse; fool" , "Cause of much head-scratching" , "Egg of parasitic insect" , "Idiot, ass" , "Young louse" .)