Black-hearted varnish maker (3)
I believe the answer is:
lac
'varnish maker' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'black-hearted' is the wordplay.
'hearted' says to take the centre.
The middle of 'black' is 'LAC'.
Can you help me to learn more?
(Other definitions for lac that I've seen before include "Catalyst" , "Huge sum" , "Red resin secreted by insects" , "Insect's resinous secretion" , "Resinous substance used in varnish" .)