Crossword Genius

Is such a brother or sister still growing? (5)

Ross

I believe the answer is:

elder

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'growing?' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for elder that I've seen before include "could be a church leader" , "Senior member of a tribe" , "White-flowered tree" , "Senior leader" , "Senior churchperson" .)

I've seen this clue in the Evening Standard.
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