Politician stops fanatic, unknown idiot (6)
I believe the answer is:
numpty
'idiot' is the definition.
(I've seen this before)
'politician stops fanatic unknown' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'stops' means one lot of letters goes inside another (stopping can mean blocking a gap).
'fanatic' becomes 'nut' (I've seen this before).
'unknown' becomes 'y' (algebra).
'mp' going into 'nut' is 'numpt'.
'numpt'+'y'='NUMPTY'
(Other definitions for numpty that I've seen before include "Fool" , "idiot from way up here" , "A Scottish idiot" .)