The most common and most likely answer to this crossword clue is the 12 letter word SHOW-STOPPER
S
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Buddy explains!

The straight part
We know that "sensation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sensation = show-stopper.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 SHOW-STOPPER mean?
SHOW-STOPPER noun- A performance or segment of a theatrical production that induces a positive audience reaction strong enough to pause the production.
- Any impediment that prevents all further progress; especially a software bug that must be fixed before any further development is possible.