What to say when taking French leave? (5)

I believe the answer is:

adieu

I'm a little stuck... Click here to teach me more about this clue!

'french leave?' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

(Other definitions for adieu that I've seen before include "Bye" , "All the best" , "Farewell (French)" , "Farewell, Monsieur!" , "12 Down" .)

I've seen this clue in the Evening Standard.

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