Copper wrong about rule - that's the key point (4)
I believe the answer is:
crux
'the key point' is the definition.
(I've seen this in another clue)
'copper wrong about rule' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'wrong' becomes 'x' (X is used to mark something as incorrect).
'about' is an insertion indicator.
'rule' becomes 'r' (legal abbreviation).
'cu'+'x'='cux'
'cux' enclosing 'r' is 'CRUX'.
'that's' acts as a link.
(Other definitions for crux that I've seen before include "Nub" , "Essence or gist" , "Vital part of a matter, decisive point" , "Most important point at issue" , "Point of difficulty" .)