Tree sap (5)
I believe the answer is:
resin
'tree sap' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for resin that I've seen before include "viscous material" , "Myrrh, for instance" , "Exudation from fir trees" , "Sticky stuff oozing from trees" , "It's obtained from tree-sap" .)