Unknown European group infiltrated by a fanatic (6)
I believe the answer is:
zealot
'fanatic' is the definition.
(I've seen this before)
'unknown european group infiltrated by a' is the wordplay.
'unknown' becomes 'z' (algebra).
'european' becomes 'e' (abbreviation e.g. EU).
'group' becomes 'lot' (lot is a kind of group).
'infiltrated by' indicates putting letters inside.
'e'+'lot'='elot'
'elot' going around 'a' is 'ealot'.
'z'+'ealot'='ZEALOT'
(Other definitions for zealot that I've seen before include "Fanatical adherent" , "Fanatic person, much too enthusiastic" , "Radical" , "Person full of enthusiasm, a fanatic" , "A fanatic or extremist" .)