Firewood (6)
(Other definitions for timber that I've seen before include "Feller's warning cry" , "22 [BEAM]" , "Deal possibly" , "Warning cry that a tree is about to fall" , "The tree's falling!" .)
(Other definitions for tinder that I've seen before include "Combustible material" , "Dry inflammable matter" , "Dry stuff to light fire with" , "It's used to start a fire" , "Dry material for starting fire" .)
'firewood?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for fir that I've seen before include "Pine tree for Gaelic men?" , "Scots pine" , "Tree for an Irishman" , "Kind of cone-bearing tree" , "Tree for Irishmen" .)