The most common and most likely answer to this crossword clue is the 6 letter word BLENNY
I've seen this crossword clue in The Daily Telegraph
B
L
E
N
N
Y
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 = blenny.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 BLENNY mean?
BLENNY noun- A true blenny, any of various marine fishes from the suborder Blennioidei or order Blenniiformes that are generally small and elongated which dwell on the sea floor, including scaled and scaleless forms and dramatically divergent appearance, in several families.
- A number of fish of similar appearance not closely related.