The most common and most likely answer to this crossword clue is the 13 letter word DEFIBRILLATOR
I've seen this crossword clue in The Times
D
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More clues leading to the result DEFIBRILLATOR
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The straight part
We know that "it'll shock you" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it'll shock you = defibrillator.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 DEFIBRILLATOR mean?
DEFIBRILLATOR noun- An electronic device used internally or externally that delivers a controlled electric shock to a patient to correct ventricular fibrillation, a lack of coordination of the contraction of muscle tissue of the large chambers of the heart.