The most common and most likely answer to this crossword clue is the 7 letter word NARWHAL
I've seen this crossword clue in The Times
N
A
R
W
H
A
L
More clues leading to the result NARWHAL
- Arctic swimmer raced north with Prince Harry
- Tusked beast smuggled back with prince
- Seen in sonar: ‘Whale … another whale‘
- Raced back with almost half a sea creature
- Warn Ahab, with line for possible catch?
- Raced back with old Prince to find aquatic mammal
- Arctic swimmer raced northward with Prince Harry
- ‘Unicorn’ worked to back up with computer from 2001
- Sea creature managed to surface with truncated sound
- Swimmer‘s nervous anticipation returned when help arrived laughing, initially
Buddy explains!

The straight part
We know that "Arctic swimmer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case arctic swimmer = narwhal.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 NARWHAL mean?
NARWHAL noun- Monodon monoceros, an Arctic cetacean that grows to about 20 feet (6 meters) long, the male having a single horn-like tusk, a twisted, pointed canine tooth that projects forward.