Is such a brother or sister still growing? (5)
I believe the answer is:
elder
'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" .)