Crossword Genius

Four, upset about expression of meditation, left - leaving probably enough people (7)

Ross

I believe the answer is:

roomful

I'm a little stuck... Click here to teach me more about this clue!

'four' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Maybe there's an association between them I don't understand?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for roomful that I've seen before include "Enough to fill a domestic space" , "As many as will fit in a domestic space" , "wall-to-wall assembly?" , "A lot of people" , "library's volume, say" .)

I've seen this clue in The Independent.
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