Roman god penning English horses (5)
I believe the answer is:
'horses' is the definition.
'roman god penning english' is the wordplay.
'roman god' becomes 'mars' (Mars is an example).
'penning' indicates putting letters inside.
'english' becomes 'E' (abbreviation).
'mars' going around 'e' is 'MARES'.
(Another definition for mares that I've seen is " Female horses".)