The most common and most likely answer to this crossword clue is the 7 letter word CARCASE
C
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The straight part
We know that "Shell" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shell = carcase.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 CARCASE mean?
CARCASE noun- The body of a dead animal.
- (meat trade) The body of a slaughtered animal, stripped of unwanted viscera, etc.
- The body of a dead human, a corpse.
- The framework of a structure, especially one not normally seen.
- An early incendiary ship-to-ship projectile consisting of an iron shell filled with saltpetre, sulphur, resin, turpentine, antimony and tallow with vents for flame.