Secretaries knocked back juice (3)
I believe the answer is:
sap
'juice' is the definition.
(juice from a plant)
'secretaries knocked back' is the wordplay.
'secretaries' becomes 'PAs' (personal assistants).
'knocked back' says the letters should be written in reverse.
'pas' written backwards gives 'SAP'.
(Other definitions for sap that I've seen before include "tunnel" , "Vital plant juice" , "Plant's fluid" , "Exhaust" , "Imbecile" .)