Rebellious Finns join a German party (4,4)
I believe the answer is:
sinn fein
'party' is the definition.
(Irish political party)
'rebellious finns join a german' is the wordplay.
'rebellious' indicates anagramming the letters.
'join' is a charade indicator (letters next to each other).
'a german' becomes 'ein' ('a' in German).
'finns' is an anagram of 'sinnf'.
'sinnf'+'ein'='SINN FEIN'
(Other definitions for sinn fein that I've seen before include "Fine inns (anag.)" , "Gerry Adams's party" , "GerryAdams's political party" , "Irish nationalist party" , "Irish political movement" .)