Tax back that might go into sawmill (5,4)
I believe the answer is:
scots pine
'tax back that might go into sawmill' is the definition.
Although both the answer and definition are singular nouns, I can'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').
'scot'+'spine'='SCOTS PINE'
Can you help me to learn more?
(Other definitions for scots pine that I've seen before include "Two-needled conifer of northern Europe" , "Kind of 3" , "One may provide deal" , "Tree common in the Highlands" , "Coniferous tree" .)