Decisive point (of the matter) (4)
I believe the answer is:
crux
(Other definitions for crux that I've seen before include "puzzle" , "Particular difficulty" , "Key point" , "Decisive point of an issue" , "Decisive, baffling, point" .)
I believe the answer is:
crux
(Other definitions for crux that I've seen before include "puzzle" , "Particular difficulty" , "Key point" , "Decisive point of an issue" , "Decisive, baffling, point" .)