Wise guy back to crack MAGA's problem with much of the Mexican border (4-3)

I believe the answer is:

know-all

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'wise guy' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for know-all that I've seen before include "Person who behaves like encyclopedia 94-3)" , "Smart alec" , "Self-styled polymath" , "Clever clogs!" , "He always has answer" .)

I've seen this clue in The Independent.

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