The most common and most likely answer to this crossword clue is the 4 letter word TUNA
I've seen this crossword clue in The Sunday Telegraph
T
U
N
A
More clues leading to the result TUNA
- A teachers’ union recalled one in school
- Swimmer, a fanatic doing the backstroke?
- Fish and a loaf brought back
- Peacekeepers infiltrating old army? Something fishy here
- Some unfortunate fish
- Head back to get a fish
- Fish fillet, unappetising portion
- . Fish head served up with aromatic starter
- . … fish in a barrel to begin with
- Swimmer, after split, ahead on the turn!
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 = tuna.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 TUNA mean?
TUNA noun- Any of several species of fish of the genus Thunnus in the family Scombridae.
- The edible flesh of the tuna.
- The prickly pear, a type of cactus native to Mexico in the genus Opuntia.
- The fruit of the cactus.