English extremists kidnap mother on river (6)
I believe the answer is:
amazon
'river' is the definition.
(Amazon is an example)
'english extremists kidnap mother on' is the wordplay.
'english extremists' becomes 'az' (the 'extreme' ends of the English alphabet).
'kidnap' means one lot of letters goes inside another.
'mother' becomes 'ma' (ma is a kind of mother).
'az' placed around 'ma' is 'amaz'.
'amaz'+'on'='AMAZON'
(Other definitions for amazon that I've seen before include "very long river" , "Aggressive female" , "S. America's longest river" , "Flower" , "Warlike female" .)