The most common and most likely answer to this crossword clue is the 7 letter word WINKLES
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More clues leading to the result WINKLES
Buddy explains!

The straight part
We know that "Shellfish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shellfish = winkles.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 WINKLES mean?
WINKLES noun- A periwinkle or its shell, of family Littorinidae.
- Any one of various marine spiral gastropods, especially, in the United States, either of two species Busycotypus canaliculata and Busycon carica.
- The penis, especially that of a boy rather than that of a man.