The most common and most likely answer to this crossword clue is the 5 letter word CONCH
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More clues leading to the result CONCH
Buddy explains!

The straight part
We know that "gastropod" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gastropod = conch.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 CONCH mean?
CONCH noun- A marine gastropod of the family Strombidae which lives in its own spiral shell.
- The shell of this sea animal.
- A musical instrument made from a large spiral seashell, somewhat like a trumpet.
- The semidome of an apse, or the apse itself.
- To refine the flavour and texture of chocolate by warming and grinding, either in a traditional concher, or between rollers.
- To play a conch seashell as a musical instrument, by blowing through a hole made close to the origin of the spiral.
- A machine (rather like a rotating pestle and mortar) used to refine the flavour and texture of chocolate by warming and grinding.
- A person who operates such a machine.