According to old soldier, Ireland has a railway fit for a horse doctor (10)
I believe the answer is:
veterinary
'doctor' is the definition.
(I know that veterinary is a type of doctor)
'according to old soldier ireland' is the wordplay.
'according' becomes 'ary' (I can't justify this - if you can you should give a lot more credence to this answer).
'to' says to put letters next to each other (I've seen this in other clues).
'old soldier' becomes 'vet' (veteran).
'ireland' becomes 'Erin' (poetic name for Ireland).
'vet'+'erin'='veterin'
'ary' put after 'veterin' is 'VETERINARY'.
'has a railway fit for a horse' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for veterinary that I've seen before include "Like some surgeons" , "Animal doctor" .)