The most common and most likely answer to this crossword clue is the 5 letter word BRILL
B
R
I
L
L
Buddy explains!

The straight part
We know that "Fab/fish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fab/fish = brill.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 BRILL mean?
BRILL noun- A type of flatfish, Scophthalmus rhombus.
- Wonderful, amusing. Denotes approval of the noun it is applied to, comparable to "cool".