The most common and most likely answer to this crossword clue is the 6 letter word SHIELD
I've seen this crossword clue in The Sunday Telegraph
S
H
I
E
L
D
More clues leading to the result SHIELD
- Something primarily gripped to protect one
- Started to carry large protective item
- Catching line, quickly thrown as safeguard
- Cast fitted round limb opening as protection
- . Started to carry large screen
- Defensive cover thrown around legionary’s head
- Screen cast getting to grips with Latin
- Screen cast, about 50
- Such defence started to hold line
- Guard started crossing line
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 = shield.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 SHIELD mean?
SHIELD noun- Anything that protects or defends; defense; shelter; protection.
- A shape like that of a shield; usually, an inverted triangle with sides that curve inward to form a pointed bottom, commonly used for police identifications and company logos.
- A large expanse of exposed stable Precambrian rock.
- A place with a toilet seat: an outhouse; a lavatory.
- Parts at the front and back of a vehicle which are meant to absorb the impact of a collision
- To protect, to defend.
- To protect from the influence of