One foolish enough to rob? (3)

I believe the answer is:

mug

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'to rob?' is the definition.
(slang)

I don't understand the remainder of the clue.

(Other definitions for mug that I've seen before include "Drinking vessel; dupe" , "Sucker - drinking vessel" , "Drinking vessel or foolish fellow" , "person's face" , "Large drinking vessel or slang victim" .)

I've seen this clue in the Evening Standard.

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