If you can play multiple notes at once on your clarinet, it'd be a grade 6 out of 1-6
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Assuming you're playing the melody notes ...
If "grade" means elementary, middle school, high school, or whatever is the UK equivalent, and assuming you've played in all grades, probably low middle school.
If "grade" means the "old style" grading of music between 1-6 for competitions in the US, with 1 being something like "Mary had a Little Lamb" and 6 being Carl Nielsen's "Concerto for Clarinet, Op. 57" (looked up on Vandoren's website for hardest clarinet pieces), I'd call this maybe a 3.
Slow pieces can be hard because you have to think about expressiveness and control over your intonation. Of course, you should also play a piece just because you like it and don't worry about how hard it's supposed to be.
Please note that if you want to play the melody line with a piano or other accompanist(s), you need to transpose the melody line.