With 46-Across, some areas in Clue . . . or a hint to the first, fourth, twelfth and fifteenth rows of this puzzle (6)
I believe the answer is:
secret
(Other definitions for secret that I've seen before include "Not known by others" , "Security classification level" , "Not to be disclosed" , "Kept under wraps" , "Confidential, not to be revealed" .)