It's a bit fishy that Gillian's lost her boy (4)

I believe the answer is:

gill

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'boy' is the definition.

I can't explain the remainder of the clue.

(Other definitions for gill that I've seen before include "A little drink" , "One quarter of a pint" , "Small liquid measure" , "Respiratory organ of fish" , "Breathing organ in fish" .)

I've seen this clue in the Evening Standard.

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