The most common and most likely answer to this crossword clue is the 6 letter word SENTRY
I've seen this crossword clue in Financial Times
S
E
N
T
R
Y
More clues leading to the result SENTRY
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 = sentry.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 SENTRY mean?
SENTRY noun- A guard, particularly on duty at the entrance to a military base.
- Sentry duty; time spent being a sentry.
- A form of drag to be towed underwater, which on striking bottom is upset and rises to the surface.
- A watchtower.