Entrant, a competitor on river (7)
I believe the answer is:
arrival
'entrant' is the definition.
(thesaurus)
'a competitor on river' is the wordplay.
'competitor' becomes 'rival' ('rival' can be a synonym of 'competitor'**).
'on' means one lot of letters go next to another.
'river' becomes 'r'.
'rival' put after 'r' is 'rrival'.
'a'+'rrival'='ARRIVAL'
(Other definitions for arrival that I've seen before include "Appearance (at a place)" , "Appearance, entrance" , "Attainment of destination" , "Appearance (at a destination)" , "Someone who's just come" .)