Data stored by apps can be cleared out by going to the iPhone Storage settings.
Go to Settings > General > iPhone Storage. Below the Recommendations section will be a list of the various apps installed on your iPhone with the ones using the most space first.
Click on the name of the app you'd like to delete the app data for. Notice the figure for Documents & Data, this includes related media such as pictures, songs and podcasts, etc.
If the Documents & Data figure is high and you're okay with loosing the existing app data, click "Delete App". Then head over to the App Store (or purchased list) and re-download it. The clean install won't include the data and documents and will take up far less space.
If you do not want to delete an apps data but still need to free up space, consider offloading the app. Offloading an app will free up space used by the app, but will keep app data. The "Offload App" setting is located in the same location referenced above.