Sole - yet not the only one (8)
I believe the answer is:
flatfish
'sole' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for flatfish that I've seen before include "Plaice or sole, for example" , "Sole, plaice etc" , "Swimmer" , "Flounder, perhaps" , "Perhaps sole" .)