Man possessed by right idea (6)
I believe the answer is:
theory
'idea' is the definition.
(I've seen this before)
'man possessed by right' is the wordplay.
'man' becomes 'he'.
'possessed by' indicates putting letters inside.
'right' becomes 'tory' (I've seen this before).
'he' inserted inside 'tory' is 'THEORY'.
(Other definitions for theory that I've seen before include "Abstract explanation" , "Idea formulated to explain something" , "Ideal situation" , "Thought" , "General principle which explains some aspect of the world" .)