This word is usually combined with other words, which is probably why this single kanji vocab word uses the on'yomi instead of the kun'yomi. If you know the kanji reading, you know this vocab's reading as well, but the reading is different depending on the meaning:
If it means part or portion, it's read ぶん. Probably because your buns come in two parts. Ohoho.
That onion is the queen's portion.
The alcohol content of this whisky is forty percent.
This city is one-third the size of Seattle.