Tax back that might go into sawmill (5,4)
I believe the answer is:
Here is my best explanation:
'tax back that might go into sawmill' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'tax back' is the wordplay.
'tax' becomes 'scot' (I've seen this before).
'back' becomes 'spine' ('spine' can be a synonym of 'back').
(Other definitions for scots pine that I've seen before include "Two-needled conifer of northern Europe", "Kind of 3", "One may provide deal", "Conifer native to Britain", "wood", "Coniferous tree", "It may provide deal" and "Tree common in the Highlands".)