Debate the cause (6)
I believe the answer is:
reason
I believe this is a double definition.
'debate' is the first definition.
(reason can mean to consider or debate)
'cause' is the second definition.
(I've seen this before)
'the' acts as a link.
(Other definitions for reason that I've seen before include "Incentive" , "Explanation; logic" , "Purpose" , "Think in a logical way" , "Mental power to justify an act" .)