The most common and most likely answer to this crossword clue is the 3 letter word DAB
I've seen this crossword clue in The Guardian
D
A
B
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 = dab.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 DAB mean?
DAB noun- A soft tap or blow; a blow or peck from a bird's beak; an aimed blow.
- A soft, playful box given in greeting or approval.
- A small amount, a blob of some soft or wet substance.
- (chiefly in the plural) Fingerprint.
- A hip hop dance move in which the dancer simultaneously drops the head while raising an arm, briefly resting their face in the elbow, as if sneezing into their elbow.
- To press lightly in a repetitive motion with a soft object without rubbing.
- To apply a substance in this way.
- To strike by a thrust; to hit with a sudden blow or thrust.
- To apply hash oil to a heated surface for the purpose of efficient combustion.
- To perform the dab dance move, by moving both arms to one side of the body parallel with your head.
- With a dab, or sudden contact.
- One skilful or proficient; an expert; an adept.
- A small flatfish of the family Pleuronectidae, especially Limanda limanda; a flounder.
- A sand dab, a small flatfish of genus Citharichthys.
- (costers) Bad.