Four, upset about expression of meditation, left - leaving probably enough people (7)
I believe the answer is:
roomful
'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" .)