Burning wood in one (2,4)
I believe the answer is:
on fire
'burning' is the definition.
(thesaurus)
'wood in one' is the wordplay.
'wood' becomes 'fir' (fir is a kind of wood).
'in' is an insertion indicator.
'fir' going within 'one' is 'ON FIRE'.
(Other definitions for on fire that I've seen before include "Enthusiastic" , "Burning" , "Excited or ablaze" , "Eager" , "Alight" .)