Sticky stuff (3)
'sticky stuff' is the definition.
(goo is a sticky substance)
This is all the clue.
(Other definitions for goo that I've seen before include "Anything sticky" , "Sticky gunge" , "Slime" , "Sticky substance (colloq.)" , "Sticky substance (sl.)" .)
(Other definitions for sap that I've seen before include "Drain of energy" , "Internal tree moisture" , "Plant juice; undermine" , "Moisture in wood of a tree" , "trench used in siege" .)