Break off the engagement (7)

I believe the answer is:

dismiss

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'break' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for dismiss that I've seen before include "Sack (from job)" , "P45" , "Sack - cashier" , "military order" , "Fire - cashier" .)

I've seen this clue in the Evening Standard.

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