Heavens! It does look fishy (8,3)

I believe the answer is:

mackerel sky

I'm a little stuck... Click here to teach me more about this clue!

'heavens' is the definition.
(mackerel sky is a kind of heavens)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for mackerel sky that I've seen before include "Rows of small white fleecy clouds" , "high-level pattern" , "Pattern of small white clouds in streaks" .)

I've seen this clue in The Globe and Mail.

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