A way of thinking or behaving (8)
I believe the answer is:
attitude
'a way of thinking or behaving' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for attitude that I've seen before include "Self-confident behaviour; way of thinking" , "Truculent behaviour" , "Stance; point of view" , "Way of thinking or behaving,stance" , "Viewpoint or posture" .)