The most common and most likely answer to this crossword clue is the 9 letter word VIGILANCE
I've seen this crossword clue in The Daily Telegraph
V
I
G
I
L
A
N
C
E
Buddy explains!
The straight part
We know that "requirement for guard" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case requirement for guard = vigilance.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 VIGILANCE mean?
VIGILANCE noun- Alert watchfulness.
- Close and continuous attention.
- A guard; a person set to watch.