Conservative welcoming the man's idea (6)
I believe the answer is:
theory
'idea' is the definition.
(theory is a kind of idea)
'conservative welcoming the man's' is the wordplay.
'conservative' becomes 'tory' (informal term for a Conservative Party member).
'welcoming' is an insertion indicator (in sense of 'inviting in').
'the man' becomes 'he'.
'tory' going around 'he' is 'THEORY'.
(Other definitions for theory that I've seen before include "Speculative view" , "General principle which explains some aspect of the world" , "Conjectural view" , "Opinion or supposition to explain something" , "Conjecture, hypothesis" .)