Tree secretion (3)
(Other definitions for gum that I've seen before include "Chewy stuff, hard to clean away" , "A chopper base" , "Part of mouth; a tree" , "Sticky plant substance" , "Adhesive - eucalyptus" .)
'tree secretion' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sap that I've seen before include "Watery substance in plants" , "The juice in trees" , "Tree's blood" , "Undermine; fool" , "Juice; energy" .)