The most common and most likely answer to this crossword clue is the 8 letter word PREFECTS
I've seen this crossword clue in Financial Times
P
R
E
F
E
C
T
S
Buddy explains!
The straight part
We know that "Monitors/old 23s" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case monitors/old 23s = prefects.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 PREFECTS mean?
PREFECTS noun- An official of Ancient Rome who controlled or superintended a particular command, charge, department, etc.
- The head of a department in France.
- The head of a prefecture in Japan.
- A school pupil in a position of power over other pupils.
- A commander.