Discharge quiet American (3)
I believe the answer is:
pus
'discharge' is the definition.
(I've seen this before)
'quiet american' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'american' becomes 'us' (abbreviation for United States).
'p'+'us'='PUS'
(Other definitions for pus that I've seen before include "Fluid formed by suppuration" , "Fluid, a product of inflammation" , "Unpleasant matter" , "Yellow matter formed in infected wound" , "Product of infected tissue" .)