How to clear cache of a specific application in iOS?
If you have installed the app directly from Xcode you can remove the data from the app in the xcode device manager.


Connect iPhone with an USB cable to your MAC
Launch Xcode
Press Shift+Cmd+2
Click on your iPhone in the list
Click on your app in the installed apps list,
Click the "Gear" icon
Click download container
Select where to store the container
Open the container remove the unwanted files, 
In Xcode device manager click replace container (Below download container)


Then the cache will be reset.