A king has challenger coming (7)
I believe the answer is:
arrival
'coming' is the definition.
(thesaurus)
'a king has challenger' is the wordplay.
'a king' becomes 'ar' (I've seen this in other clues**).
'has' means one lot of letters go next to another.
'challenger' becomes 'rival' ('rival' can be a synonym of 'challenger').
'ar'+'rival'='ARRIVAL'
(Other definitions for arrival that I've seen before include "Attainment of destination" , "The A in ETA" , "Entry" , "Advent" , "Appearance (at a destination)" .)