Ambassador took part in conservative speculation (6)
I believe the answer is:
theory
'speculation' is the definition.
(I've seen this before)
'ambassador took part in conservative' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'took part in' indicates putting letters inside.
'conservative' becomes 'tory' (informal term for a Conservative Party member).
'he' put within 'tory' is 'THEORY'.
(Other definitions for theory that I've seen before include "Idea for testing" , "Opinion or supposition to explain something" , "Conjecture, hypothesis" , "One with specialised knowledge" , "Scientific conjecture" .)