Fluid in plants (3)
I believe the answer is:
sap
'fluid 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 "Gradually weaken (a person's strength)" , "The juice in trees" , "Plant juice for silly ass" , "Weak foolish person" , "Plant juice; undermine" .)