Black hardwood (5)
I believe the answer is:
ebony
'black hardwood' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ebony that I've seen before include "Very dark brown" , "'Hard, dark wood (5)'" , "Tree having black wood" , "Hard dark-coloured wood" , "Tree with blackish wood" .)