Crux, gist (3)
I believe the answer is:
nub
I believe this clue is a double definition.
'crux' is the first definition.
(I've seen this before)
'gist' is the second definition.
(synonyms)
(Other definitions for nub that I've seen before include "Most essential part of an idea" , "Crux, gist" , "Point or gist" , "Centre, heart" , "Central point or core of matter" .)