The most common and most likely answer to this crossword clue is the 13 letter word AGGLOMERATION
I've seen this crossword clue in The Times
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More clues leading to the result AGGLOMERATION
Buddy explains!
The straight part
We know that "mass" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mass = agglomeration.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 AGGLOMERATION mean?
AGGLOMERATION noun- The act or process of collecting in a mass; a heaping together.
- State of being collected in a mass; a mass; cluster.
- An extended city area comprising the built-up area of a central city and any suburbs linked by continuous urban area.
- A mass of large volcanic fragments bonded under heat.