Bucking up a number in audition (10)

I believe the answer is:

heartening

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'bucking up' is the definition.
The definition and answer can be both to do with human feelings as well as being present participle verbs.
Maybe they are linked in a way I don't understand?

'a number in audition' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'in' means one lot of letters goes inside another.
'audition' becomes 'hearing' (synonyms).
'ten' going inside 'hearing' is 'HEARTENING'.

Can you help me to learn more?

(Another definition for heartening that I've seen is " Cheering".)

I've seen this clue in the Sydney Morning Herald.

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