Crossword Genius

Cups for gullible people (4)

Ross

I believe the answer is:

mugs

I'm a little stuck... Click here to teach me more about this clue!

'gullible people' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for mugs that I've seen before include "Drinking cups" , "'Cups, faces and fall guys (4)'" , "Attacks" , "Faces (sl.)" , "Cylindrical cups without saucer" .)

I've seen this clue in The Mirror.
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