The Ancient Mariner, say, returns for Arabian big fish (4)

I believe the answer is:

emir

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'fish' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see an association between them that I can't see?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for emir that I've seen before include "Local chief" , "Islamic chieftain" , "leader in the East" , "Arab leader" , "Eastern ruler in mire" .)

I've seen this clue in The Guardian.

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