He’s of service in getting a horse out (6)
I believe the answer is:
marine
'he's of service' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'getting a horse out' is the wordplay.
'getting' becomes 'in' (I can't explain this - if you can you should believe this answer much more).
'a horse' becomes 'mare' (mare is a kind of horse).
'out' indicates putting letters inside.
'in' inserted inside 'mare' is 'MARINE'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for marine that I've seen before include "Oceanic" , "Concerned with the sea" , "Maritime - seagoing soldier" , "Airmen (anag.)" , "Sea-soldier" .)