Japanese dish — keep quiet about American one (5)
I believe the answer is:
sushi
'japanese dish' is the definition.
(I've seen this before)
'keep quiet about american one' is the wordplay.
'keep quiet' becomes 'sh'.
'about' is an insertion indicator.
'american' becomes 'us'.
'one' becomes 'i' (Roman numeral).
'sh' going around 'us' is 'sush'.
'sush'+'i'='SUSHI'
(Other definitions for sushi that I've seen before include "cold food" , "Japanese vinegared rice dish" , "Fated" , "Asian food" , "Japanese food" .)