Breaking clusters in 8-ball is an important part of winning, and there is more than one way to do it. One way is to simply the run the cue ball into them and continue your run. Another way is to break them up by playing safe. Look at the following example:
We are stripes, and the 9-10 have no clear pocket. We could pocket the 14 and run into the cluster. This is a bit risky, we may not have a good shot afterwards. Another option is to break them up by playing safe and locking up our opponent tight.
Not only is our cluster open, our opponent must take a low-percentage kick shot. We will likely get the table back, and from here we can easily run out.
Always look at your clusters closely: see if there is a way to make an easy stop-shot to both open the cluster and lock your opponent up against your ball with no clear shot. This is a very strong safety play in 8-ball.