He is trained in horse jumping (7)
I believe the answer is:
gymnast
'he is trained in horse jumping' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for gymnast that I've seen before include "Agile athlete" , "My angst over indoors athlete" , "he may be in for the high jump" , "Agile exerciser" , "Athlete in the skill of strength, balance and agility" .)