One sister has run away to get engaged (2,3)

I believe the answer is:

in use

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'engaged' is the definition.
(synonyms)

I can't explain the rest of the clue.

(Other definitions for in use that I've seen before include "Current" , "Being employed - unavailable" , "Currently employed" , "taken?" , "prevailing" .)

I've seen this clue in The Times.

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