Hi all - bored at home in Covid Quarantine - and stumbled on this thread. I'm a professional clarinetist - www.paulsaundersclarinet.co.uk -and thought I'd share my thoughts.
Gliss-ing is tricky one to master and is only usually utilised from c (above middle c) to the top (rhapsody in blue) c. Having said that, it's possible to go further as the gif demonstrates.
The way in to this effect is to imagine 'catching' the sound at the back of your throat using your larynx in the same way a thumb might go over a water tap to make a jet. If done correctly, this will strangle the sound, it doesn't sound good but it's not supposed to.
Try this on an 'all fingers down' C. When you hear the change the next step is to make a break in the sound.
Under normal circumstances this would have a massive detrimental affect on the note BUT because you have 'caught' the sound at the back of the throat you 'should' begin to gain control of the beginning of a gliss.
Initially, lift the RH 4th (ring) finger whilst playing the C and see if that works. If it doesn't, then you haven't successfully 'caught' the sound.
If you have, CONGRATULATIONS !
Then experiment with lifting the middle and then the index finger. Once the 'break' in the sound has been made, you should be able to gliss anywhere within the 2 octave range, providing your embouchure can already find those notes.
https://www.dropbox.com/s/zuze94g24ajvlj4/Quarantine%20Glissing%20Lesson%20Vid.mov?dl=0
Hope that helps.
Paul