Gullible person sticks up gum (3)

I believe the answer is:

mug

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'gullible person' is the definition.
(mug can mean a gullible or foolish person)

'sticks up gum' is the wordplay.
'gum' can be anagrammed to 'MUG'.
But, I'm not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for mug that I've seen before include "person's face" , "simpleton" , "A person easily deceived, by gum" , "Fool - drinking vessel" , "Drinking vessel for a chump" .)

I've seen this clue in The Mirror.

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