Plain to see when caught out in secret (5)

I believe the answer is:

overt

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'plain to see' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for overt that I've seen before include "Open and observable" , "Open and candid" , "Done in an obvious way" , "Trove (anag.)" , "Openly displayed" .)

I've seen this clue in The Telegraph.

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