The most common and most likely answer to this crossword clue is the 6 letter word CREELS
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The straight part
We know that "containers for fish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case containers for fish = creels.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 CREELS mean?
CREELS noun- An osier basket, such as anglers use to hold fish.
- A bar or set of bars with skewers for holding paying-off bobbins, as in the roving machine, throstle, and mule.