Colt's mother (4)
I believe the answer is:
'colt's mother' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mare that I've seen before include "For classicists, sea" , "Female equine animal" , "Ream (anag)" , "Horse; lunar sea" , "A night-time strangler" .)