Dare To, Presume To, Daringly
adverb
This is the adverb version of the kanji daring, so it means dare to or daringly.
あえて
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You haven't learned this reading yet so here's a mnemonic to help you: You're about to daringly jump off a cliff and you shout, "AAAHHH (あ)!" to get your courage up! You always have to yell AAHH before you dare to do daring things!
あなただからこそ、敢えて厳しいことを言わせてもらったんだよ。
I only dared to say something so intense because it was to you.