well culture clashes are generally, as the name gives away, when two different cultures disagree between each other. many films cover this, you just have to think carefully. you could try and be clever and see the culture clashes in films depicting wars, although to be honest with you, if i was given your homework, i'd just do Alien vs. Predator.
oh, a very good use of culture clash, is the film "Quadrophenia", look that up.
