The most common and most likely answer to this crossword clue is the 9 letter word UNTRAINED
I've seen this crossword clue in The Daily Telegraph
U
N
T
R
A
I
N
E
D
More clues leading to the result UNTRAINED
Buddy explains!

The straight part
We know that "Lacking discipline" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lacking discipline = untrained.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 UNTRAINED mean?
UNTRAINED adjective- Lacking training, not having been instructed in something.