Juice dad’s knocked over (3)
I believe the answer is:
sap
'juice' is the definition.
(juice from a plant)
'dad's knocked over' is the wordplay.
'dad's' becomes 'pa's' (both can mean one's father).
'knocked over' says the letters should be written in reverse.
'pas' written backwards gives 'SAP'.
(Other definitions for sap that I've seen before include "Cosh; plant's juice" , "Plant liquid or general vitality" , "Drain of energy" , "Weak foolish person" , "Plant juice; undermine" .)