Trained horse, regularly nervy, to submit (7)
I believe the answer is:
eventer
'trained horse' is the definition.
(horse trained for equestrian events)
'regularly nervy to submit' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'to submit' becomes 'enter' (as in to submit something to a competition).
The alternating letters of 'nervy' are 'ev'.
'ev'+'enter'='EVENTER'
(Other definitions for eventer that I've seen before include "Jumper" , "Equestrian horse" , "competitive horse" , "Competing horse or rider" , "Competitive rider" .)