Could mostly act deviously, unable to keep a secret (6)
I believe the answer is:
occult
'a secret' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
(Other definitions for occult that I've seen before include "Esoteric, magical" , "Involving magical powers" , "Secret - paranormal" , "Secret - supernatural" , "Mystical, involving the supernatural" .)