Rider to join after European victory (7)
I believe the answer is:
eventer
'rider' is the definition.
(I have seen 'Competitive rider ' mean 'eventer' so perhaps 'rider' could also mean 'eventer')
'join after european victory' is the wordplay.
'join' becomes 'enter' (I've seen this in other clues).
'after' means one lot of letters go next to another.
'european' becomes 'E' (abbreviation e.g. EU).
'victory' becomes 'v'.
'e'+'v'='ev'
'enter' after 'ev' is 'EVENTER'.
'to' is the link.
(Other definitions for eventer that I've seen before include "versatile horse" , "Competing horse or rider" , "Equestrian horse" , "Rider in contests" , "competitive horse" .)