The most common and most likely answer to this crossword clue is the 8 letter word WATCHMAN
I've seen this crossword clue in The Times
W
A
T
C
H
M
A
N
Buddy explains!

The straight part
We know that "Guard" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case guard = watchman.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 WATCHMAN mean?
WATCHMAN noun- One set to watch; a person who keeps guard, especially one who guards a building, or the streets of a city, by night.