Lumber source (4)
I believe the answer is:
tree
'lumber source' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tree that I've seen before include "Genealogist's work" , "Be up a .... to be in difficulties" , "box, possibly" , "trod the boards" , "Beech, for example" .)