Ambassador taken in by right-wing supposition (6)
I believe the answer is:
theory
'supposition' is the definition.
(I've seen this before)
'ambassador taken in by right-wing' is the wordplay.
'ambassador' becomes 'HE' (His or Her Excellency).
'taken in by' means one lot of letters goes inside another (back).
'right-wing' becomes 'Tory'.
'he' going within 'tory' is 'THEORY'.
(Other definitions for theory that I've seen before include "Thought" , "Series of ideas seeking to explain something" , "Justifying hypothesis" , "Scientific supposition" , "Proposed explanation of something" .)