One of the fruits of advanced age? (10)
I believe the answer is:
elderberry
'one of the fruits' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'advanced age?' is the wordplay.
I cannot really see how this works, but
'age?' could be 'eld' (I have seen 'Old age' mean 'eld' so perhaps 'age' could also mean 'eld') and 'eld' is present in the answer.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for elderberry that I've seen before include "A blue-black fruit" , "Fruit used for cordial" , "Fruit used for jelly or wine" , "Fruit used for home-made wine" , "Winemaker" .)