To the Romans, a sea horse? (4)
I believe the answer is:
mare
'horse?' is the definition.
(I know that mare is a type of horse)
'to the romans a sea' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
The remaining letters 'mre' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for mare that I've seen before include "Female equine animal" , "A night-time strangler" , "Female quadruped" , "Adult female horse" , "Horse; lunar sea" .)