Mare crossing a river? (3)
I believe the answer is:
dam
'mare' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'crossing a river?' is the wordplay.
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'river?' becomes 'dm' (I can't explain this - if you can you should give a lot more credence to this answer).
'a' inserted into 'dm' is 'DAM'.
Can you help me to learn more?
(Other definitions for dam that I've seen before include "Wall restraining water" , "Embankment" , "Female parent of an animal, especially livestock" , "Block (river)" , "Weir" .)