Cause, justification (6)
I believe the answer is:
reason
This clue is a double definition.
'cause' is the first definition.
(I've seen this before)
'justification' is the second definition.
(I've seen this in another clue)
(Other definitions for reason that I've seen before include "Employ logic" , "Explanation; logic" , "Justification (for action)" , "Reach logical conclusions" , "Cause; think logically" .)