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