Encourage satiety and good spirits (12)

I believe the answer is:

cheerfulness

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'good spirits' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'encourage satiety' is the wordplay.
'encourage' becomes 'cheer' (cheering is a kind of encouraging).
'satiety' becomes 'fulness' (I can't explain this - if you can you should believe this answer much more).
'cheer'+'fulness'='CHEERFULNESS'

'and' acts as a link.

Can you help me to learn more?

I've seen this clue in The Globe and Mail.

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