Giving a guarantee, which is auspicious (9)

I believe the answer is:

promising

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'auspicious' is the definition.
(similar in meaning)

'giving a guarantee which is' is the wordplay.
I cannot really see how this works, but
'is' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for promising that I've seen before include "Giving one's word" , "Favourable -- hopeful" , "Encouraging" , "Showing great potential" .)

I've seen this clue in the Evening Standard.

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