Star swimmer (6)
I believe the answer is:
mullet
This clue might be a double definition.
'star' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'swimmer' is the second definition.
(mullet is a kind of aquatic vertebrate)
Can you help me to learn more?
(Other definitions for mullet that I've seen before include "Barnet?" , "Food fish - hairstyle" , "Starfish" , "Fish, male hairstyle" , "Fish (red or grey)" .)