The most common and most likely answer to this crossword clue is the 6 letter word ZANDER
I've seen this crossword clue in The Guardian
Z
A
N
D
E
R
Buddy explains!

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 ZANDER mean?
ZANDER noun- A European freshwater fish in the family Percidae, closely related to the perch, Sander lucioperca.
- Any fish of the genus Sander that live in freshwater.