Separate Volume
noun
Usually you call books "volumes" when they come in sets, right? This is like that, but separate, making this a separate volume.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. The べつ gets shortened to べっ, though, probably because it's a つ and that's what つs do for a living in jukugo words.
私はこのコミックの別冊を集めています。
I collect the separate editions of this comic.
母は私に別冊少年ジャンプを買ってくれました。
My mother bought me a separate issue of Shonen Jump.
別冊の表紙のモデル、見た？めっちゃカッコいいよ！
Did you see the model on the cover of the other book? It's really cool!