One with a maternal interest in American youth (3)
I believe the answer is:
mom
This clue could be a double definition.
'one' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'a maternal interest in american youth' is the second definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'with' acts as a link.
Can you help me to learn more?
(Other definitions for mom that I've seen before include "A familiar American mother" , "'US mother, informally (3)'" , "US mother, in short" .)