Knocked over small stool, silly me! (4)
I believe the answer is:
oops
'silly me' is the definition.
(both might be said after making a mistake)
'knocked over small stool' is the wordplay.
'knocked over' shows that the letters should be reversed in order.
'small' becomes 's' (abbreviation - e.g. clothes size).
'stool' becomes 'poo' (both can mean excrement).
's'+'poo'='spoo'
'spoo' in reverse letter order is 'OOPS'.
(Other definitions for oops that I've seen before include "Oh dear, I've spilt something!" , "Something's been spilt!" , "Oh dear, that's a mistake!" , "Exclamation on making a minor mistake" , "Exclamation recognising a mistake has been made" .)