Swimmer describing a sky (8)
I believe the answer is:
mackerel
'swimmer' is the definition.
(mackerel is a kind of fish)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for mackerel that I've seen before include "that's fishy" , "A fish; sort of breeze, sky" , "Banded food fish" , "A food fish; type of sky" , "Food fish with a blue-green back" .)