Volume used by US associate in prosaic way (9)
I believe the answer is:
literally
'in prosaic way' is the definition.
(using words prosaically rather than figuratively)
'volume used by us associate' is the wordplay.
'volume used by us' becomes 'liter' (US spelling of litre).
'associate' becomes 'ally' (ally is a kind of associate).
'liter'+'ally'='LITERALLY'
(Other definitions for literally that I've seen before include "precisely as written" , "According to the letter, exactly" , "Absolutely" , "According to the precise meaning of words" , "Without exaggeration" .)