Political group not completely unknown (5)
I believe the answer is:
party
'political group' is the definition.
(a political party)
'not completely unknown' is the wordplay.
'not completely' becomes 'part' ('part' is the opposite to 'completely').
'unknown' becomes 'y' (algebra).
'part'+'y'='PARTY'
(Other definitions for party that I've seen before include "Political organisation" , "Political organization" , "See 22" , "Social gathering; person" , "Celebratory social gathering" .)