Foal's mother (4)
I believe the answer is:
mare
'foal'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 "Adult female horse" , "Nag" , "A night-time strangler" , "For classicists, sea" , "Ream (anag)" .)