Trained horse always to skirt opening (7)
I believe the answer is:
eventer
'trained horse' is the definition.
(horse trained for equestrian events)
'always to skirt opening' is the wordplay.
'always' becomes 'eer' (poetic way of saying ever).
'to skirt' indicates putting letters inside (skirt can mean to go around).
'opening' becomes 'vent' (a vent is an opening or outlet).
'eer' placed around 'vent' is 'EVENTER'.
(Other definitions for eventer that I've seen before include "Equestrian competitor" , "Competitive rider" , "Competing horse or rider" , "Rider in contests" , "versatile horse" .)