Appearance of artist upset opponent (7)
I believe the answer is:
arrival
'appearance' is the definition.
(to appear can mean to arrive at a particular place)
'artist upset opponent' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'upset' says the letters should be written backwards.
'opponent' becomes 'rival' (a rival is an opponent or enemy).
'ra' reversed gives 'ar'.
'ar'+'rival'='ARRIVAL'
'of' is the link.
(Other definitions for arrival that I've seen before include "coming on the scene" , "Coming - comer" , "Advent" , "The reaching of one's destination" , "Appearance (at a destination)" .)