Leader in equestrianism, victory key for competitive horse (7)
I believe the answer is:
eventer
'competitive horse' is the definition.
(I've seen this before)
'leader in equestrianism victory key' is the wordplay.
'leader in' suggests taking the first letters.
'key' becomes 'enter' (key on a computer keyboard).
The initial letters of 'equestrianism victory' is 'ev'.
'ev'+'enter'='EVENTER'
'for' acts as a link.
(Other definitions for eventer that I've seen before include "Jumper" , "competitive horse" , "Rider in contests" , "Showjumper" , "Competing horse or rider" .)