The most common and most likely answer to this crossword clue is the 5 letter word GAPER
I've seen this crossword clue in The Guardian
G
A
P
E
R
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 = gaper.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 GAPER mean?
GAPER noun- One who gapes; a starer.
- Any of several species of burrowing clams.
- The comber, a fish of the species Serranus cabrilla.
- The fish Champsodon capensis
- An inexperienced skier.