The ambassador is captivated by the Conservatives’ principles (8)
I believe the answer is:
theories
'principles' is the definition.
(I've seen this before)
'the ambassador is captivated by the conservatives'' is the wordplay.
'the ambassador' becomes 'he' (His or Her Excellency).
'is captivated by' means one lot of letters goes inside another.
'the conservatives'' becomes 'tories' (Tory is an example).
'he' inserted within 'tories' is 'THEORIES'.
(Other definitions for theories that I've seen before include "Explanatory suppositions" , "Abstract principles" , "Hypotheses, possibilities" , "General principles or opinions" , "premises?" .)