Sanity; cause or motive (6)
I believe the answer is:
reason
I believe this clue is a double definition.
'sanity' is the first definition.
(reason is a kind of sanity)
'cause or motive' is the second definition.
(I've seen this before)
(Other definitions for reason that I've seen before include "Sanity and logic" , "Power of thought" , "Defence" , "Motive; argue" , "logical argument" .)