Rider happening to join late queen (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')
'happening to join late queen' is the wordplay.
'happening' becomes 'event' (I've seen this before).
'to join' is a charade indicator (letters next to each other).
'late queen' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
'event'+'er'='EVENTER'
(Other definitions for eventer that I've seen before include "Competitive horse/rider" , "Showjumper" , "Competitive rider" , "Equestrian horse" , "Sportsman" .)