The most common and most likely answer to this crossword clue is the 14 letter word FUNDOPLICATION
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The straight part
We know that "gathering in theatre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gathering in theatre = fundoplication.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 FUNDOPLICATION mean?
FUNDOPLICATION noun- An operation in which the gastric fundus (upper part) of the stomach is wrapped, or plicated, around the lower end of the esophagus and stitched in place, reinforcing the closing function of the lower esophageal sphincter. The esophageal hiatus is also narrowed down by sutures to prevent or treat concurrent hiatal hernia, in which the fundus slides up through the enlarged esophageal hiatus of the diaphragm.