It’s Google that’s replaced scientist (9)
I believe the answer is:
geologist
'scientist' is the definition.
(geologist is a kind of scientist)
'it's google that's replaced' is the wordplay.
'that's replaced' indicates an anagram.
'its'+'google'='itsgoogle'
'itsgoogle' is an anagram of 'GEOLOGIST'.
(Other definitions for geologist that I've seen before include "Specialist in the earth's history recorded in rocks" , "One studying the earth's crust" , "One studying the earth's structure" , "Specialist in the physical history of the earth" , "One studying rocks" .)