Visually check the belt for signs of glazing - check the pullys as well.
The belt is probably loose, but a loose belt can glaze pullys, and cause the new belt to squeal as well. Glazing is easy to fix - take some scotch-brite to it. If the belt is glazed, replace it.
The clutch will usually slip when it starts going bad - if it's going too long, the rivets in the friction disc will start to gouge the cover and flywheel. As long as your clutch is good and solid, it's probably still good. About once a month, I do a second gear takeoff to see how well it's grabbing - if I can do a solid, smooth takeoff in second, it's usually still good.
Gear noise would be constant while you are moving for the most part - if the squeal comes when you blip the throttle in neutral, it's definitely an engine noise (the best way to save hours when troubleshooting a noise is to eliminate things one piece at a time, and then you know where to look...)
Oh - and just to help you a little - "squeaking", "squealing", and "accelerating." Most of us here will only know you by your writing - so you'll want to present a sound image. Fortunately (for me, at least!) most of us here can also tell the difference between bad writing and typographical errors (it's only taken me 22 years to learn to type!) I just want to help you get the best you can - and present yourself in the best way possible.
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