Opponent supporting a king's appearance (7)
I believe the answer is:
arrival
'appearance' is the definition.
(to appear can mean to arrive at a particular place)
'opponent supporting a king's' is the wordplay.
'opponent' becomes 'rival' (I've seen this before).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'a king' becomes 'ar' (I've seen this in other clues).
'rival' put after 'ar' is 'ARRIVAL'.
(Other definitions for arrival that I've seen before include "Appearance (at a destination)" , "coming on the scene" , "Entry" , "Coming - comer" , "Appearance (at a place)" .)