It’s not okay to keep your senpai waiting.
How much longer will I have to wait?
Sorry for making you wait so long. I had to wait for the bus for such a long time.
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means wait so this word has something to do with waiting. Use the たせ as a hint. Someone "tossed" a ball off into a ditch, and then they had to find it. They made you wait for them to get the ball too, even though it's a cheap piece of crap. Feel upset about this.
You learned this reading from 待つ, congrats!