a short fight between two armies or groups - used in news reports:
armed clashes along the border
an argument between two people or groups because they have very different beliefs or opinions - used in news reports
The plans put oil companies in a head-on clash with environmentalists.
personality/culture clash (=a situation in which two people or groups do not like each other)
a sports match between two teams, players etc that is expected to be very exciting - used in sports reports:
The heavyweight clash goes ahead in Las Vegas on 8 May.
a situation in which two events happen at the same time in a way that is inconvenient:
a scheduling clash on TV
a loud sound made by two metal objects hitting each other:
the clash of swords
a combination of two colours, designs etc that look bad together:
a colour clash