Cause for sanity (6)
I believe the answer is:
reason
I believe this is a double definition.
'cause' is the first definition.
(I know that cause can be written as reason)
'sanity' is the second definition.
(reason is a kind of sanity)
'for' is the link.
(Other definitions for reason that I've seen before include "Incentive" , "Defence" , "Employ logic" , "Cause, explanation" , "issue" .)