Roman god penning English horses (5)
I believe the answer is:
mares
'horses' is the definition.
(female horses)
'roman god penning english' is the wordplay.
'roman god' becomes 'mars' (Mars is an example).
'penning' is an insertion indicator.
'english' becomes 'E' (abbreviation as in OED).
'mars' enclosing 'e' is 'MARES'.
(Another definition for mares that I've seen is " Female horses".)
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