Horse holding one girl (5)
I believe the answer is:
marie
'girl' is the definition.
'horse holding one' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'holding' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'mare' enclosing 'i' is 'MARIE'.
(Other definitions for marie that I've seen before include "girl" , "- Lloyd, English music- hall entertainer" , "-- Antoinette" , "-- Curie, scientist" , "French queen, d.1793" .)