Principal part of the brain (8)
I believe the answer is:
cerebrum
'principal part of the brain' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cerebrum that I've seen before include "not a whole loaf?" , "Principal part of brain in front of skull" , "One thinks" , "Principle part of brain, in front of skull" , "Main part of the brain" .)