Clash on film is when different individuals or groups have different ideas about what they want to create in a movie and these ideas clash or conflict with each other during the process of making the movie.
Think of it like playing with blocks with your friends. You might want to build a big tower of blocks, but your friend wants to build a bridge. These two ideas clash because they are very different. You might argue with your friend and it might be hard to decide what to do. The same thing happens with movies.
Sometimes, the director of the movie has one idea, the actors have their own ideas, and the producers have another idea. They might argue with each other about what should happen in the movie, what the characters should say and how they should act. It can be very difficult to make everyone happy.
This clash can lead to things like scenes being cut from the movie or actors leaving the project, which can change the entire story. In some cases, the movie might even be cancelled altogether.
So just like trying to build something with blocks, making a movie can sometimes be tricky when everyone has their own ideas, and it can result in some disagreements and difficulties.