Composing mission for Kentish marriage bureau? (8)

I believe the answer is:

sedating

'composing mission' is the definition.
'sedating' can be an answer for 'composing' (sedate can mean to calm or compose). I'm unsure of the 'mission' bit.

'kentish marriage bureau?' is the wordplay.
'kentish' becomes 'SE' (South East).
'marriage bureau?' becomes 'dating' (I can't explain this - if you can you should believe this answer much more).
'se'+'dating'='SEDATING'

'for' acts as a link.

(Another definition for sedating that I've seen is " calming down".)

I've seen this clue in The Times.

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