Juice the old man's knocked over (3)
I believe the answer is:
sap
'juice' is the definition.
(thesaurus)
'the old man's knocked over' is the wordplay.
'the old man's' becomes 'pas' ('pa' can be a synonym of 'old man').
'knocked over' shows that the letters should be reversed in order.
'pas' backwards is 'SAP'.
(Other definitions for sap that I've seen before include "Weak foolish person" , "Fluid in plant stems" , "It's plant juice, idiot" , "dig tunnels" , "Moisture in wood of a tree" .)