Watery substance in plants (3)
I believe the answer is:
sap
'watery substance in plants' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sap that I've seen before include "Exhaust" , "Plant juice; undermine" , "Vigour, energy" , "Moisture in wood of a tree" , "dig tunnels" .)