Plant fluid (3)
'plant fluid' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sap that I've seen before include "dig tunnels" , "Watery substance in plants" , "Fluid in plant stems" , "Undermine; fool" , "Enfeeble" .)
(Other definitions for latex that I've seen before include "Milky substance produced by some trees" , "Stretchy stuff" , "Synthetic product" , "Whitish milky fluid" , "Juice in rubber-making" .)