Mother concerned with horse (4)
I believe the answer is:
mare
'horse' is the definition.
(mare is a kind of horse)
'mother concerned with' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'concerned with' becomes 're'.
'ma'+'re'='MARE'
(Other definitions for mare that I've seen before include "Ream (anag)" , "Female equine animal" , "For classicists, sea" , "A night-time strangler" , "Adult female zebra" .)