Crossword Genius

Assignation shown to be impossible? (7)

Ross

I believe the answer is:

mission

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'assignation' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the rest of the clue.

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(Other definitions for mission that I've seen before include "religious establishment" , "In religious campaign" , "Vocation; embassy" , "Assignment; expedition" , "Delegation" .)

I've seen this clue in the Evening Standard.
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