Vintage unknown - that's the essential point (4)
I believe the answer is:
crux
'the essential point' is the definition.
(I've seen this before)
'vintage unknown' is the wordplay.
'vintage' becomes 'cru' (e.g. grand cru).
'unknown' becomes 'x' (algebra).
'cru'+'x'='CRUX'
'that's' is the link.
(Other definitions for crux that I've seen before include "Essence or gist" , "Pivotal point - cross" , "Baffling problem" , "The most important thing" , "Key point" .)