Countless millions milk supplier put up (6)
I believe the answer is:
myriad
'countless' is the definition.
(I know that countless can be written as myriad)
'millions milk supplier put up' is the wordplay.
'millions' becomes 'm' (abbreviation for millions).
'milk supplier' becomes 'dairy' (building which processes cow's milk).
'put up' shows that the letters should be reversed in order.
'dairy' backwards is 'yriad'.
'm'+'yriad'='MYRIAD'
(Other definitions for myriad that I've seen before include "Host" , "A great number" , "Countless - very large but indeterminate number" , "Too numerous to be counted" , "Immense number found by dim ray" .)