Mature minor Liberal back to play eternally (9)

I believe the answer is:

agelessly

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'eternally' is the definition.
Both the answer and definition are adverbs.
Maybe you can see a link between them that I don't see?

'mature minor liberal back to play' is the wordplay.
'mature' becomes 'age' (synonyms).
'minor' becomes 'less' (I've seen this before).
'liberal' becomes 'l' (abbreviation).
'back to' says to take the final letters.
The last letter of 'play' is 'y'.
'age'+'less'+'l'+'y'='AGELESSLY'

Can you help me to learn more?

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

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