The most common and most likely answer to this crossword clue is the 6 letter word WRASSE
I've seen this crossword clue in The Guardian
W
R
A
S
S
E
More clues leading to the result WRASSE
- River, Charlie? You and I will catch that fish
- Fish with twit on banks
- Fish with idiot on banks
- Swimmer was seen around river Seine's banks
- Colourful swimmer may see poor waitress without it
- Fish wars breaking out with some excitement to begin with
- Colourful swimmer swears outrageously
- Fish river area, visiting West Sussex regularly
- The two of us saving money after chopping head off fish
- Take shells from water, anxious, and see fish
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 = wrasse.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 WRASSE mean?
WRASSE noun- Any one of numerous edible, marine, spiny-finned fishes of the family Labridae, of which several species are found in the Mediterranean and on the Atlantic coast of Europe. Many of the species are brightly colored.