The most common and most likely answer to this crossword clue is the 11 letter word AGGREGATION
I've seen this crossword clue in The Times
A
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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 = aggregation.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 AGGREGATION mean?
AGGREGATION noun- The act of collecting together (aggregating).
- The state of being collected into a mass, assemblage, or sum (aggregated).
- A collection of particulars; an aggregate.
- Summarizing multiple routes into one route.
- The majority of the parasite population concentrated into a minority of the host population.
- Kind of object composition which does not imply ownership.