The most common and most likely answer to this crossword clue is the 6 letter word ASTERN
I've seen this crossword clue in The Guardian
A
S
T
E
R
N
More clues leading to the result ASTERN
- Seat wrecked by sailors towards the rear
- A small seabird away to the rear
- Part of coaster nearly at the back
- A second bird at the back of the ship
- Seeing that schooner, following ship
- A southern bird to the rear
- Situated towards the back, like a bird
- Behind schoolmaster, necking drinks
- A second seabird on the back of a liner, say
- New seat given to sailors behind
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 = astern.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 ASTERN mean?
ASTERN adjective- Behind a vessel; having a bearing of 180 degrees from ahead.
- Behind (a vessel); in the rear.
- In the direction of the stern; backward (motion); to the rear.
- At or toward the rear of a vessel.