angry, bitter, fierce, serious | armed, bloody, violent | head-on
The leaders are preparing for a head-on clash at the summit.

culture, personality

lead to

break out

in a/the ~
Several people were injured in violent clashes with the police.

~ between
Clashes between the rebels and government forces have broken out in the north.

~ of
a clash of opinions

~ over
Differences in the aims of the two unions have led to serious clashes over policy.

~ with

a clash of cultures/interests/wills

violently | repeatedly

on/over
He has clashed repeatedly with the team coach over training schedules.

with
They often clash violently with rival gangs.