As is 'billet-dou' in 'a billet-doux'? (6,4,1,4)

I believe the answer is:

sealed with a kiss

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'billet-doux'?' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'as is billet-dou in a' is the wordplay.
I cannot quite see how this works, but
'is' is within the answer.
'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

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I've seen this clue in The Guardian.

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