Numberless mass milk producer upset (6)
I believe the answer is:
myriad
This clue could be a double definition.
'numberless' is the first definition.
(synonyms)
'mass milk producer upset' is the second definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps there's a link between them I don't understand?
Can you help me to learn more?
(Other definitions for myriad that I've seen before include "immense quantity" , "Countless, extremely large number" , "An indefinitely large number" , "Numberless" , "Many" .)