The most common and most likely answer to this crossword clue is the 7 letter word SAILING
I've seen this crossword clue in The Daily Telegraph
S
A
I
L
I
N
G
More clues leading to the result SAILING
- Saint sick in a boat?
- Sons in poor health — would this bring on nausea?
- What one likes doing with son in a bad way
- Voyage in trouble east of Sweden
- When doing this, initially seasick?
- Unwell below decks, finally at sea
- At the mercy of wind, feeling sick after the last of oysters
- Some passengers ailing go by boat
- Sport requiring excellent pitch outside
- Son's sick on the move at sea
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 = sailing.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 SAILING mean?
SAILING verb- To be impelled or driven forward by the action of wind upon sails, as a ship on water; to be impelled on a body of water by steam or other power.
- To move through or on the water; to swim, as a fish or a waterfowl.
- To ride in a boat, especially a sailboat.
- To set sail; to begin a voyage.
- To move briskly and gracefully through the air.
- To move briskly.
- Travelling by ship.
- Motion across a body of water in a craft powered by the wind, as a sport or otherwise
- Navigation; the skill needed to operate and navigate a vessel
- The time of departure from a port
- (countable) a scheduled voyage by a ferry or ship.