The man taken in by Conservative's surmise (6)
I believe the answer is:
theory
'surmise' is the definition.
(thesaurus)
'the man taken in by conservative's' is the wordplay.
'the man' becomes 'he'.
'taken in by' means one lot of letters goes inside another.
'conservative' becomes 'tory' (I've seen this before).
'he' inserted inside 'tory' is 'THEORY'.
(Other definitions for theory that I've seen before include "Justifying hypothesis" , "Idea to explain something, say evolution" , "Supposition intended to explain observations" , "Suggested explanation" , "Principle on which practice is based" .)