The most common and most likely answer to this crossword clue is the 10 letter word ILLITERATE
I've seen this crossword clue in The Daily Telegraph
I
L
L
I
T
E
R
A
T
E
More clues leading to the result ILLITERATE
- Poor English grade after that subject is not taught
- I will repeat for one with language difficulties
- Large American volume by don that is unable to write
- Dunce’s unfortunate to say it again
- Dunce‘s unfortunate to say it again
- I half-read a little, sadly lacking education?
- Characters backward in alphabet are, till informed!
- Unlike readers, the writer will perform repeatedly
- I will repeat: ‘That’s ignorant!’
- Uneducated crook in Sydney? Say again
Buddy explains!

The straight part
We know that "lacking education" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lacking education = illiterate.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ILLITERATE mean?
ILLITERATE noun- An illiterate person, one not able to read and write.
- A person ignorant about a given subject.
- Unable to read and write.
- Having less than an expected standard of familiarity with language and literature, or having little formal education.
- Not conforming to prescribed standards of speech or writing.
- Ignorant in a specified way or about a specified subject.