To Lack Something
transitive verb, godan verb
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 lack, and this verb means to lack something. Pretty simple in English, but 欠く is actually kind of advanced and formal in Japanese, so it's mostly just used in writing.
Also keep in mind this is a transitive verb, so 欠く takes a direct object — maybe someone lacks knowledge, for example, or you lack concentration. This is a bit different from saying "your concentration is lacking," at least grammatically. (For that, you'd use 欠ける.)
You learned this reading when you learned 欠ける, but here's a mnemonic just in case:
You never know when you're going to lack something. Like a car (か). Did you plan for it to break? NO! But it happened and now you lack a car in a big way. Not cool (く)!
バナナだけの食事はバランスを欠いていると思いますよ。
I think your banana-only diet lacks balance.
生の魚は、買った三日後には、どうしてもフレッシュさを欠きますね。
Raw fish can't help but lack freshness three days after purchase.
インターネットは私たちの生活に欠くことのできないサービスだ。
The internet is an indispensable service in our lives.