Specialist in the history of the earth in rocks (9)
I believe the answer is:
geologist
'specialist in the history of the earth in rocks' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for geologist that I've seen before include "One studying rocks" , "Specialist in the physical history of the earth" , "One studying the earth's crust" , "One studying the earth's structure" , "Specialist in the history of the earth in rocks" .)