Countermarch annoys mendacious extremists, causing deep fissures (6)
I believe the answer is:
chasms
'deep fissures' is the definition.
(I've seen this in another clue)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for chasms that I've seen before include "Deep fissures in the earth" , "Deep gorges, crevasses" , "Deep fissure in the earth" , "Big gaps" , "Deep openings in the earth's surface" .)