The most common and most likely answer to this crossword clue is the 7 letter word LAMPREY
I've seen this crossword clue in The Daily Telegraph
L
A
M
P
R
E
Y
More clues leading to the result LAMPREY
- Aquatic creature hit hard, animal killed by another
- Fish in batter animal hunted as food
- Hit on victim, sucker
- Beat victim, a sucker
- Sucker punch on victim
- Soldiers hungry at the end after light seafood
- Marine carnivore mounts attack on quarry
- Swimmer seen in large empty aquarium before game
- Hit on nose saving English swimmer that wriggles
- Pound put on game, often losing? Sucker
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 = lamprey.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 LAMPREY mean?
LAMPREY noun- Any long slender primitive eel-like freshwater and saltwater fish of the order Petromyzontiformes, having a sucking mouth with rasping teeth but no jaw.