The most common and most likely answer to this crossword clue is the 6 letter word COCKLE
I've seen this crossword clue in Independent
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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 = cockle.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 COCKLE mean?
COCKLE noun- Any of various edible European bivalve mollusks, of the family Cardiidae, having heart-shaped shells.
- The shell of such a mollusk.
- (in the plural) One’s innermost feelings (only in the expression “the cockles of one’s heart”).
- (directly from French coquille) A wrinkle, pucker
- (by extension) A defect in sheepskin; firm dark nodules caused by the bites of keds on live sheep
- The mineral black tourmaline or schorl.
- The fire chamber of a furnace.
- A kiln for drying hops; an oast.
- The dome of a heating furnace.
- To cause to contract into wrinkles or ridges, as some kinds of cloth after a wetting; to pucker.
- Any of several field weeds, such as the common corncockle (Agrostemma githago) and darnel ryegrass (Lolium temulentum).
- A £10 note; a tenner.