Briefly study issue with some common sense (6)
I believe the answer is:
Here is my best explanation:
'some common sense' is the definition.
'briefly study issue' is the subsidiary indication.
'briefly' means to remove the last letter.
'study' becomes 'read' ('read' can be a synonym of 'study').
'issue' becomes 'son'.
'read' with its last letter taken off is 'rea'.
'with' is the link.
(Other definitions for reason that I've seen before include "Motivation or cause", "Cause for sanity", "Capacity for rational thought", "Sanity of senora", "Logic; excuse", "Sense", "Rationality", "Sanity, the ability to think", "Cause; think logically" and "Motive for an action".)