One gathers wood for burning (2,4)
I believe the answer is:
on fire
'burning' is the definition.
(thesaurus)
'one gathers wood' is the wordplay.
'gathers' indicates putting letters inside.
'wood' becomes 'fir' (fir is a kind of wood).
'one' going around 'fir' is 'ON FIRE'.
'for' is the link.
(Other definitions for on fire that I've seen before include "Alight" , "in flames" , "Excited or ablaze" , "Eager" , "Enthusiastic" .)