Endlessly casting about, catching high-class marine creature (3,6)

I believe the answer is:

sea urchin

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'marine creature' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for sea urchin that I've seen before include "marine echinoderm" , "Main thing" , "Marine invertebrate - enrich USA (anag)" , "spineless creature" , "Echinoderm with a thin spiny globular shell" .)

I've seen this clue in The Times.

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