The most common and most likely answer to this crossword clue is the 4 letter word LOST
I've seen this crossword clue in The Guardian
L
O
S
T
More clues leading to the result LOST
- Behold, good person’s gone astray!
- Beyond redemption at sea
- Were defeated, upsetting lots
- Completely mislaid or misplaced, essentially?
- & 2 Abandoned lay: the failed would-be MP’s forfeit
- & 19 Suffered defeat: perplexed, crushed
- Many turning tail never to be found
- Regularly looks at TV series
- Finished at the bottom, like 8’s tail
- Misplaced loyalty often sustains tyrannical leaders
Buddy explains!

The straight part
We know that "at sea" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case at sea = lost.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 LOST mean?
LOST verb- To cause (something) to cease to be in one's possession or capability due to unfortunate or unknown circumstances, events or reasons.
- To wander from; to miss, so as not to be able to find; to go astray from.
- To fail to win (a game, competition, trial, etc).
- To be unable to follow or trace (somebody or something) any longer.
- To cause (somebody) to be unable to follow or trace one any longer.
- To cease exhibiting; to overcome (a behavior or emotion).
- To shed, remove, discard, or eliminate.
- Of a clock, to run slower than expected.
- (ditransitive) To cause (someone) the loss of something; to deprive of.
- To fail to catch with the mind or senses; to miss.
- To cause to part with; to deprive of.
- Having wandered from, or unable to find, the way.
- In an unknown location; unable to be found.
- Not perceptible to the senses; no longer visible.
- Parted with; no longer held or possessed.
- Not employed or enjoyed; thrown away; employed ineffectually; wasted; squandered.
- Ruined or destroyed, either physically or morally; past help or hope.
- Hardened beyond sensibility or recovery; alienated; insensible.
- Occupied with, or under the influence of, something, so as not to notice external things.