Shifting Earl to the north, Napoleon gains key wood (5)
I believe the answer is:
ebony
'wood' is the definition.
(ebony is a kind of wood)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ebony that I've seen before include "hard stuff" , "Black hardwood" , "Very dark black" , "Hard wood used for black piano keys and in cabinetwork" , "Material for key" .)