Rob of a vessel? (3)

I believe the answer is:

mug

I believe this clue is a double definition.

'rob' is the first definition.
(slang)

'vessel?' is the second definition.
(mug is a kind of vessel)

'of a' is the link.

(Other definitions for mug that I've seen before include "Charlie" , "Gullible person" , "Drinking vessel, by gum" , "Slangy victim, by gum" , "Beaker" .)

I've seen this clue in the Evening Standard.

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