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 cannot understand how they can define each other.
'black-hearted' is the wordplay.
'hearted' indicates the central letters.
The centre 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" .)