Joke about tree burning (2,4)
I believe the answer is:
on fire
'burning' is the definition.
(something on fire is burning or ablaze)
'joke about tree' is the wordplay.
'joke' becomes 'one' (as in 'that's a good one!').
'about' indicates putting letters inside.
'tree' becomes 'fir' (fir tree is a kind of tree).
'one' going around 'fir' is 'ON FIRE'.
(Other definitions for on fire that I've seen before include "Alight" , "Excited or ablaze" , "Eager" , "Burning" , "in flames" .)