The most common and most likely answer to this crossword clue is the 12 letter word SELF-CONTROL
I've seen this crossword clue in The Times
S
E
L
F
-
C
O
N
T
R
O
L
Buddy explains!
The straight part
We know that "discipline" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case discipline = self-control.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 SELF-CONTROL mean?
SELF-CONTROL noun- The ability to control one's desires and impulses; willpower.