Man with a mare (4)
I believe the answer is:
adam
'man' is the definition.
(man's name)
'with a mare' is the wordplay.
'with' says to put letters next to each other.
'mare' becomes 'dam' (I can't explain this - if you can you should believe this answer much more).
'a' next to 'dam' is 'ADAM'.
Can you help me to learn more?
(Other definitions for adam that I've seen before include "One of our common ancestors?" , "who transformed Palladian architecture?" , "neoclassical style" , "Male" , "Eden man" .)