Countless millions to back milk producer (6)

I believe the answer is:

myriad

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'countless' is the definition.
('myriad' can be a synonym of 'countless')

I don't understand the rest of the clue.

(Other definitions for myriad that I've seen before include "Many" , "Large indefinite number" , "Host" , "Umpteen" , "a vast number, perhaps innumerable" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).

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