The most common and most likely answer to this crossword clue is the 12 letter word ACCUMULATION
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The straight part
We know that "Gathering" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gathering = accumulation.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 ACCUMULATION mean?
ACCUMULATION noun- The act of amassing or gathering, as into a pile.
- The process of growing into a heap or a large amount.
- A mass of something piled up or collected.
- The concurrence of several titles to the same proof.
- The continuous growth of capital by retention of interest or savings.
- The action of investors buying an asset from other investors when the price of the asset is low.
- The practice of taking two higher degrees simultaneously, to reduce the length of study.