The most common and most likely answer to this crossword clue is the 12 letter word SCHOOLMASTER
I've seen this crossword clue in The Guardian
S
C
H
O
O
L
M
A
S
T
E
R
More clues leading to the result SCHOOLMASTER
- Might one organise the classroom?
- One bringing order to the classroom?
- The classroom in turmoil? His responsibility!
- Fish joining stream after swimming instructor
- Teacher rearranged the classroom
- He upsets some scholar about the beginning of term
- He organises room with satchels scattered
- The classroom is untidy — and he’s responsible
- Teacher reorganized the classroom
Buddy explains!

The straight part
We know that "instructor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case instructor = schoolmaster.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 SCHOOLMASTER mean?
SCHOOLMASTER noun- A male teacher.
- Male teacher in charge of a school, usually a small one.
- Anything that teaches.
- To teach in the capacity of schoolmaster.