Cause; grounds (6)
I believe the answer is:
reason
I believe this clue is a double definition.
'cause' is the first definition.
(I've seen this before)
'grounds' is the second definition.
(I've seen this before)
(Other definitions for reason that I've seen before include "Sanity, the ability to think" , "Justification; sanity" , "Logic; excuse" , "Cause or motive" , "Employ logic" .)