Gum will recede on gullible person (3)

I believe the answer is:

mug

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

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

'gum will recede' is the wordplay.
'gum' anagrams to 'MUG'.
But, I am not clear how the anagram is indicated.

'on' acts as a link.

Can you help me to learn more?

(Other definitions for mug that I've seen before include "concentrate on studying" , "Large cup without a saucer" , "Charlie" , "Sucker - drinking vessel" , "attack" .)

I've seen this clue in The Mirror.

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