One snide eccentric republican - he's well-placed (7)
I believe the answer is:
insider
'he's well-placed' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one snide eccentric republican' is the wordplay.
'one' becomes 'i' (Roman numeral).
'eccentric' indicates an anagram.
'republican' becomes 'R' (political abbreviation).
'snide' with letters rearranged gives 'nside'.
'i'+'nside'+'r'='INSIDER'
Can you help me to learn more?
(Other definitions for insider that I've seen before include "member of the club" , "Member of a specific group" , "One with special knowledge" , "One privy to info by being within organisation" , "One privy to restricted information" .)