The most common and most likely answer to this crossword clue is the 10 letter word GATEKEEPER
I've seen this crossword clue in The Daily Telegraph
G
A
T
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K
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P
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More clues leading to the result GATEKEEPER
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 = gatekeeper.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 GATEKEEPER mean?
GATEKEEPER noun- A person or group who controls access to something or somebody.
- A person who guards or monitors passage through a gate.
- A common orange and brown butterfly with eyespots, Pyronia tithonus, of the family Nymphalidae.