Join king getting nothing back (4)
I believe the answer is:
link
'join' is the definition.
(I know that join is a more specific form of the action link)
'king getting nothing back' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'getting' is a charade indicator (letters next to each other).
'nothing' becomes 'nil' (eg nil is zero in football).
'back' says the letters should be written backwards.
'nil' back-to-front is 'lin'.
'k' after 'lin' is 'LINK'.
(Other definitions for link that I've seen before include "Tie-up" , "Relationship; intertwine" , "Connect" , "Join; loop in a chain" , "Relationship; loop in a chain" .)