Key material (5)
I believe the answer is:
ebony
'key material' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ebony that I've seen before include "Hard wood used for black piano keys and in cabinetwork" , "Tree having black wood" , "Very dark brown" , "Very dark black" , "Hard block tropical wood" .)