Tree is what burns, largely (3)
I believe the answer is:
fir
'tree' is the definition.
(fir tree is a kind of tree)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for fir that I've seen before include "Kind of cone-bearing tree" , "Tree for an Irishman" , "Pine tree for Gaelic men?" , "wood" , "Coniferous tree" .)