Fabulously fishy fellow (6)

I believe the answer is:

merman

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'fishy fellow' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

(Other definitions for merman that I've seen before include "fishy character" , "Fabulous creature" , "Ethel --, singer" , "Half man, half fish" .)

I've seen this clue in the Evening Standard.

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