Per Process for reviewing classic confinement snaps, part of the request for classic is to describe why classic is needed. In the case of clementine, I would suspect it would work just fine as a strict snap. We have interfaces for binding to a well-known dbus name (dbus interface), network, home (for accessing ~/Music), avahi-abserve (eg, daap), etc, etc. If you used one of the desktop snapcraft parts, I suspect this would be fairly straightforward.
Why specifically does clementine need to be classic? Have you used
sudo snappy-debug.security scanlog (first
snap install snappy-debug) under devmode or strict mode to see what interfaces to connect?