Expert on the rocks? (9)
I believe the answer is:
geologist
'expert on the rocks?' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps there's an association between them I don't understand?
This is the entire clue.
Can you help me to learn more?
(Other definitions for geologist that I've seen before include "One studying rocks" , "Specialist in the earth's history recorded in rocks" , "One studying the earth's crust" , "Specialist in the history of the earth in rocks" , "One studying the earth's structure" .)