Guess thought to be false (7)

I believe the answer is:

suspect

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'guess' is the definition.
(suspecting is a kind of guessing)

I cannot understand how the remainder of the clue works.

(Other definitions for suspect that I've seen before include "Dubious" , "Have qualms about" , "Believe (someone) guilty" , "Presume" , "Believe (something) possible or probable" .)

I've seen this clue in The Sun.

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