Regularly leave key equestrian competitor (7)
I believe the answer is:
eventer
'equestrian competitor' is the definition.
(I've seen this before)
'regularly leave key' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'key' becomes 'enter' (key on a computer keyboard).
The alternating letters of 'leave' are 'ev'.
'ev'+'enter'='EVENTER'
(Other definitions for eventer that I've seen before include "versatile horse" , "Jumper" , "Competitive rider" , "Competitive horse/rider" , "Competing horse or rider" .)