Border controlled by China is most important (6)
I believe the answer is:
primal
'most important' is the definition.
(I've seen this in another clue)
'border controlled by china' is the wordplay.
'border' becomes 'rim' (both can mean an edge).
'controlled by' means one lot of letters goes inside another (some letters restrict or block in others).
'china' becomes 'pal' (china plate is Cockney rhyming slang for mate).
'rim' placed into 'pal' is 'PRIMAL'.
'is' is the link.
(Other definitions for primal that I've seen before include "Primitive or fundamental" , "First; primitive" , "Sort of scream" , "Relating to an early stage of evolution" , "Having existed from the beginning" .)