Crossword Genius

Cher may open this, which prevents issue (6,6)


I believe the answer is:

french letter

'which prevents issue' is the definition.
(a condom, which prevents offspring or issue)

I cannot understand how the rest of the clue works.

(Another definition for french letter that I've seen is " Contraceptive (slang)".)

I've seen this clue in The Guardian.
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