In CLion v2016.1.1, when you ctrl+mouse-click on a function it will open a new tab in which you can find the function declaration (this works if function usage and declaration/definition are in differents file).

To come back you only have to close the tab.

Instead, if you want to come back to a previous view in the same file you can select from menu Navigate > Back.
There is also a shortcut ctrl+alt+← (left), but there is problem with it: it won't work because it is caught by Unity when you want to change a workspace. If you want to use a shortcut you can change the keybinding in CLion preferences. To do so follow these steps:

1. Open keymap preference: from menu select Edit > Settings... > Keymap
2. Look for back in the search bar.
3. Watch at results and on Navigate entry right-click and choose Add Keyboard Shortcut
4. Fill the new window with your preferred shortcut and select OK

Now you should be able to use the shortcut to come back to a previous view.

I hope this could help you.