That man involved in conservative speculation (6)
I believe the answer is:
theory
'speculation' is the definition.
(thesaurus)
'that man involved in conservative' is the wordplay.
'that man' becomes 'he'.
'involved in' indicates putting letters inside.
'conservative' becomes 'tory' (I've seen this before).
'he' going inside 'tory' is 'THEORY'.
(Other definitions for theory that I've seen before include "Series of ideas seeking to explain something" , "Assumption" , "can provide an explanation" , "Explanation or system of anything" , "Supposition or idea" .)