Royal imprisoning current extremists in powerful rule (9)
I believe the answer is:
principle
'rule' is the definition.
(I know that principle is a type of rule)
'royal imprisoning current extremists in powerful' is the wordplay.
'royal' becomes 'prince' (prince is a kind of royal).
'imprisoning' means one lot of letters goes inside another.
'current' becomes 'I' (symbol for electric current).
'extremists in' suggests removing the centre.
'powerful' with its middle removed is 'pl'.
'i'+'pl'='ipl'
'prince' going around 'ipl' is 'PRINCIPLE'.
(Other definitions for principle that I've seen before include "Precept, rule" , "Morally correct behaviour" , "General scientific law" , "Law of nature, rule of conduct" , "In which, theoretically" .)