Border where mother has drink with king (6)
I believe the answer is:
margin
'border' is the definition.
(I know that border can be written as margin)
'mother has drink with king' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'has' means one lot of letters go next to another.
'drink' becomes 'gin' (gin is an alcoholic drink).
'with' is a charade indicator (letters next to each other).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'gin' after 'r' is 'rgin'.
'ma' next to 'rgin' is 'MARGIN'.
'where' is the link.
(Other definitions for margin that I've seen before include "Permissible difference" , "Edge, border, fringe" , "Edge; room for error" , "Bound" , "Border or leeway" .)