One gets a horse out for her (5)
I believe the answer is:
marie
'her' is the definition.
'one gets a horse out' is the wordplay.
'one' becomes 'I' (Roman numeral).
'gets a horse' becomes 'mare' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'out' means one lot of letters goes inside another.
'i' put within 'mare' is 'MARIE'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for marie that I've seen before include "- Lloyd, English music- hall entertainer" , "girl" , "Queen, guillotined in 1793" , "-- Antoinette" , "French queen, d.1793" .)