The most common and most likely answer to this crossword clue is the 8 letter word FLATFISH
I've seen this crossword clue in Independent
F
L
A
T
F
I
S
H
More clues leading to the result FLATFISH
Buddy explains!

The straight part
We know that "Flounder, perhaps" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flounder, perhaps = flatfish.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 FLATFISH mean?
FLATFISH noun- A fish of the order Pleuronectiformes, the adults of which have both eyes on one side and usually swim with the other side down, such as a flounder, a halibut, or a sole.