Halter quickly flipped over mare's back (5)
I believe the answer is:
noose
'halter' is the definition.
(thesaurus)
'quickly flipped over mare's back' is the wordplay.
'quickly' becomes 'soon' (I've seen this in other clues).
'flipped over' shows that the letters should be reversed in order.
'back' says to take the final letters.
The last letter of 'mare' is 'e'.
'soon' backwards is 'noos'.
'noos'+'e'='NOOSE'
(Other definitions for noose that I've seen before include "Circular loop" , "Catcher" , "Catch" , "Halter loop" , "Snare, restraint" .)