The most common and most likely answer to this crossword clue is the 5 letter word GUARD
I've seen this crossword clue in The Daily Telegraph
G
U
A
R
D
Buddy explains!
The straight part
We know that "behind" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case behind = guard.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 GUARD mean?
GUARD noun- A person who, or thing that, protects or watches over something.
- A garda; a police officer.
- A squad responsible for protecting something.
- The part of a sword that protects the wielder's hand.
- A part of a machine which blocks access to dangerous parts.
- A watchchain.
- A panel of a car that encloses the wheel area, especially the front wheels.
- A state of caution; posture of defence.
- Something worn to protect part of the body, e.g. the shins in cricket.
- A relatively short player, playing farther from the basket than a forward or center.
- The position on the popping crease where a batsman makes a mark to align himself with the wicket; see take guard.
- Either of two offensive positions between the center and each of the offensive tackles, whose main responsibilities are to protect the quarterback, and open up "holes" through which offensive players can run.
- A player playing a position named guard.
- An employee, normally travelling in the last vehicle of a train, responsible for the safety of the train.
- A Boolean expression that must evaluate to true for a branch of program execution to continue.
- To protect from danger; to secure against surprise, attack, or injury; to keep in safety; to defend.
- To keep watch over, in order to prevent escape or restrain from acts of violence, or the like.
- To watch by way of caution or defense; to be caution; to be in a state or position of defense or safety.
- To protect the edge of, especially with an ornamental border; hence, to face or ornament with lists, laces, etc.
- To fasten by binding; to gird.