Conservative to accept governor’s hypothesis (6)
I believe the answer is:
theory
'hypothesis' is the definition.
('theory' can be a synonym of 'hypothesis')
'conservative to accept governor's' is the wordplay.
'conservative' becomes 'tory' (I've seen this before).
'to accept' means one lot of letters goes inside another.
'governor' becomes 'he' (I've seen this in other clues).
'tory' enclosing 'he' is 'THEORY'.
(Other definitions for theory that I've seen before include "Working hypothesis" , "The words, as opposed to the practice" , "Opinion or supposition to explain something" , "Explanation or system of anything" , "General principles, as opposed to practice" .)