He stops Conservative speculation (6)
I believe the answer is:
theory
'speculation' is the definition.
(I've seen this before)
'he stops conservative' is the wordplay.
'stops' is an insertion indicator.
'conservative' becomes 'tory' (informal term for a Conservative Party member).
'he' put into 'tory' is 'THEORY'.
(Other definitions for theory that I've seen before include "Idea formulated to explain something" , "The words, as opposed to the practice" , "Hypothesis - doctrine" , "Suggested explanation" , "General principles, as opposed to practice" .)