Stone cottage in Brittany occupied by artist (northern) (7)
I believe the answer is:
granite
'stone' is the definition.
(I know that granite is a type of batholite)
'cottage in brittany occupied by artist northern' is the wordplay.
'cottage in brittany' becomes 'gite' (I am not sure about this - if you are sure you should believe this answer much more).
'occupied by' indicates putting letters inside.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'ra'+'n'='ran'
'gite' going around 'ran' is 'GRANITE'.
Can you help me to learn more?
(Other definitions for granite that I've seen before include "Very hard type of rock" , "Tearing (anag.)" , "Get rain (anag) - rock" , "determination" , "Showing great resolution" .)