Join Left in Korea’s capital (4)
I believe the answer is:
link
'join' is the definition.
('link' can be a synonym of 'join')
'left in korea's capital' is the wordplay.
'left' becomes 'l' (common abbreviation).
'capital' suggests taking the first letters.
The first letter of 'korea' is 'k'.
'l'+'in'+'k'='LINK'
(Other definitions for link that I've seen before include "Tie-up" , "Connexion" , "Connecting part" , "Relationship; intertwine" , "Item of mail" .)