The most common and most likely answer to this crossword clue is the 7 letter word ESCOLAR
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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 = escolar.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 ESCOLAR mean?
ESCOLAR noun- Lepidocybium flavobrunneum, one of the snake mackerels.
- Any fish of species Ruvettus pretiosus, oilfish.
- Any of several other perciform fish of the family Gempylidae (snake mackerels).