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

culture, personality

lead to, provoke
avoid

break out, erupt, occur

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

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

clash of
a clash of opinions

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

clash with

a clash of cultures, a clash of interests, a clash of wills

violently
frequently, repeatedly

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

with
They often clash violently with rival gangs.

Clash is used with these nouns as the subject:
army, colour, metal, protester, side

Clash is used with these nouns as the object:
cymbal