Crossword Genius

Fool one holding drink (3)

Ross

I believe the answer is:

mug

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'fool' is the definition.
(I know that fool can be written as mug)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for mug that I've seen before include "Drinking vessel; rob" , "simpleton" , "Large cup without a saucer" , "In slang, a person easily deceived" , "Gullible person" .)

I've seen this clue in The Sun.
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