The most common and most likely answer to this crossword clue is the 6 letter word MINDER
I've seen this crossword clue in Financial Times
M
I
N
D
E
R
More clues leading to the result MINDER
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 = minder.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 MINDER mean?
MINDER noun- One who minds, tends, or watches something such as a child, a machine, or cattle; a keeper
- A personal bodyguard.
- A guide assigned by the authorities to foreign visitors so as to exercise control over their contacts with the populace.
- One who is taken care of, such as a pauper child in the care of private person; a ward.