clash

clash 

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if two armies, groups etc clash, they start fighting - used in news reports 

Troops clashed near the border 

clash with۱ 
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Police have clashed with demonstrators again today  

clash with۲ 

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 if two colours or designs clash, they look very bad together

I can't wear red - it clashes with my hair 

clash over/on  

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The two men have clashed over the report's conclusions