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 "Sanity, the ability to think", "Cause, explanation", "brain operations", "Cause for sanity", "An explanation", "Motivation or justification", "Think in a logical way", "use of wit", "Sanity and logic" and "Logic; excuse".)