Cause, motive (6)
I believe the answer is:
reason
This clue is a double definition.
'cause' is the first definition.
(I know that cause can be written as reason)
'motive' is the second definition.
(I know that reason is a type of rational motive)
(Other definitions for reason that I've seen before include "Think in a logical way" , "Cause; think logically" , "Motivation or cause" , "brain operations" , "Sanity and logic" .)