Reasoning, logic (9)
I believe the answer is:
rationale
This clue is a double definition.
'reasoning' is the first definition.
(I've seen this before)
'logic' is the second definition.
(I've seen this before)
(Other definitions for rationale that I've seen before include "justification" , "Motivation" , "Good reason" , "The logical basis of some thing or action" , "grounds?" .)