If the tone of your clarinet has lost something (clarity) or you are now finding that your left hand upper register is flat and it it is harder to focus the sound in the g-c range then check your register key tube.
It could be dirty or corroded inside If the instrument is fairly new, probably a pipe cleaner perhaps dipped in a little cleaner will work, don't scratch the bore though.
To be really cleaned especially if there is corrosion inside it needs to be removed from the instrument.(A job for the repairman as you could ruin your instrument if you make a mistake like trying to lever it out, especially if is one of the screw in type)
Reducing the diameter of the tube will lower notes( altissimo and upper left hand (clarion) and throat Bb) enlarging it will raise them ( this will also effect your throat Bb which might get too diffuse if the tube is too big))
A lot of tuning issues are related to this tube. It can be shortened to raise the upper left hand and its taper can be altered.. How high your register key opens will also effect tuning (more open will raise the pitch and change the feel of the clarinet) The register tube on the A clarinet usually sticks into the clarinet more than the Bb one does. If the hole in the tube was big enough to make a good Bb the upper register would not speak. As this tube functions as a tone hole it is shaped somewhat like the upper tone holes with a choke.
While you are peering down, there have a look at the thumb tube, hopefully the curve in the inside of the tube runs from side to side rather than end to end. Aligning this improves the clarinet. Many new clarinets have this defect.