The most common and most likely answer to this crossword clue is the 3 plus 4 letter word SEA BASS
I've seen this crossword clue in The Guardian
S
E
A
B
A
S
S
More clues leading to the result SEA BASS
- Swimmer with the deep, deep voice
- Home Counties singer consuming a marine fish
- Swimmer with deep voice?
- In audition, pick out instrument that might be found in school
- As discussed, observe deep fish
- Sole gutted, a low fish
- Swimmer with the deep voice?
- Great Olympian catching leader in aquatics, American behind swimmer
- Soprano with energy, a singer and swimmer
- South European sailor on a ship gets fish
Buddy explains!

The straight part
We know that "fish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fish = sea bass.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.