Fellow's trendy idea about British rule (6)
I believe the answer is:
member
'fellow's' is the definition.
(thesaurus)
'trendy idea about british rule' is the wordplay.
'trendy idea' becomes 'meme' (I am not sure about this - if you are sure you should believe this answer much more).
'about' indicates putting letters inside.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'rule' becomes 'r' (legal abbreviation).
'meme' enclosing 'b' is 'membe'.
'membe'+'r'='MEMBER'
Can you help me to learn more?
(Other definitions for member that I've seen before include "Limb; society joiner" , "Part of the body - or of society group" , "Person belonging to a particular club or society" , "Part of body or one who belongs to a club" , "Eg, leg, arm" .)