School dance I arrange, almost guaranteeing party (8)

I believe the answer is:

promisor

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

'almost guaranteeing party' is the definition.
I can't judge whether this definition defines the answer.

'school dance i arrange' is the wordplay.
'school dance' becomes 'prom' (I've seen this in another clue).
'arrange' becomes 'sor' (I can't explain this - if you can you should believe this answer much more).
'prom'+'i'+'sor'='PROMISOR'

Can you help me to learn more?

(Another definition for promisor that I've seen is " Undertaker".)

I've seen this clue in The Times.

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