The most common and most likely answer to this crossword clue is the 8 plus 3 letter word ELECTRIC RAY
I've seen this crossword clue in The Daily Telegraph
E
L
E
C
T
R
I
C
R
A
Y
More clues leading to the result ELECTRIC RAY
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 = electric ray.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.