First man to have a mare (4)
I believe the answer is:
adam
'first man' is the definition.
(first man in the Bible)
'a mare' is the wordplay.
'mare' becomes 'dam' (I can't justify this - if you can you should believe this answer much more).
'a'+'dam'='ADAM'
'to have' acts as a link.
Can you help me to learn more?
(Other definitions for adam that I've seen before include "Biblical first man" , "One of our common ancestors?" , "Man on the ceiling of the Sistine Chapel" , "A mad sort of ancestor" , "Father of man" .)