The most common and most likely answer to this crossword clue is the 9 letter word SPECIALTY
I've seen this crossword clue in The Times
S
P
E
C
I
A
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Y
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 = specialty.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 SPECIALTY mean?
SPECIALTY noun- That in which one specializes; a chosen expertise or talent.
- Particularity.
- A particular or peculiar case.
- An attribute or quality peculiar to a species.
- A contract or obligation under seal; a contract by deed; a writing, under seal, given as security for a debt particularly specified.