These fish-eaters are fools (5)

I believe the answer is:

gulls

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'fools' is the definition.
('gull' can be a synonym of 'fool')

'these fish-eaters' is the wordplay.
'these fish' becomes 'gull' (synonyms).
'eaters' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'gull'+'s'='GULLS'

'are' acts as a link.

Can you help me to learn more?

(Other definitions for gulls that I've seen before include "Seabirds - dupes" , "Aquatic birds" , "mugs" .)

I've seen this clue in The Sun.

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