An example of 15 extremists in Canada and US, twitching (7)
I believe the answer is:
caustic
'an example of 15 extremists' is the definition.
I can't tell whether this definition defines the answer.
'canada and us twitching' is the wordplay.
'canada' becomes 'ca' (I am not sure about this - if you are sure you should believe this answer much more).
'and' is a charade indicator (letters next to each other).
'twitching' becomes 'tic' (tic is a kind of twitching).
'ca'+'us'+'tic'='CAUSTIC'
'in' is the link.
Can you help me to learn more?
(Other definitions for caustic that I've seen before include "Burning, corroding" , "Harsh or corrosive" , "Severe" , "Burning - cutting" , "Corrosive - cutting" .)