The most common and most likely answer to this crossword clue is the 12 letter word OVERCROWDING
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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 OVERCROWDING mean?
OVERCROWDING verb- To fill beyond reasonable limits, with people, animals, objects or information.
- The situation where a space holds more occupants than it can comfortably accommodate.