Dad's knocked over fluid (3)
I believe the answer is:
sap
'fluid' is the definition.
(fluid from plants)
'dad's knocked over' is the wordplay.
'dad's' becomes 'pa's' ('pa' can be a synonym of 'dad').
'knocked over' says the letters should be written in reverse.
'pas' back-to-front is 'SAP'.
(Other definitions for sap that I've seen before include "Plant's fluid" , "Drain - vital fluid" , "Vigour, energy" , "Undermine the tree juice" , "Gradually weaken (a person's strength)" .)