Essence (sometimes hard) (4)
I believe the answer is:
core
(Other definitions for core that I've seen before include "Crux" , "Heart of the matter" , "Heal" , "Drilled rock sample" , "Inmost part" .)
I believe the answer is:
core
(Other definitions for core that I've seen before include "Crux" , "Heart of the matter" , "Heal" , "Drilled rock sample" , "Inmost part" .)