That fellow is captivated by Conservative system of ideas (6)
I believe the answer is:
theory
'system of ideas' is the definition.
(a theory is a system of ideas)
'that fellow is captivated by conservative' is the wordplay.
'that fellow' becomes 'he' ('he' refers to a man).
'is captivated by' means one lot of letters goes inside another.
'conservative' becomes 'tory' (Tory means a member of the Conservative Party).
'he' placed within 'tory' is 'THEORY'.
(Other definitions for theory that I've seen before include "Ideal situation" , "Opinion or supposition to explain something" , "Proposed explanation of something" , "General principles, as opposed to practice" , "abstract reasoning" .)