American tucking into his exotic Japanese dish (5)
I believe the answer is:
sushi
'japanese dish' is the definition.
(I've seen this before)
'american tucking into his exotic' is the wordplay.
'american' becomes 'us'.
'tucking into' means one lot of letters goes inside another.
'exotic' is an anagram indicator.
'his' is an anagram of 'shi'.
'us' put within 'shi' is 'SUSHI'.
(Other definitions for sushi that I've seen before include "Rice and raw fish wrapped in seaweed" , "Oriental food" , "oriental dish" , "exotic dish" , "Raw fish dishes" .)