Celebrate sushi finally being in list (7)
I believe the answer is:
roister
'celebrate' is the definition.
(I've seen this before)
'sushi finally being in list' is the wordplay.
'sushi finally' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'being in' indicates putting letters inside.
'list' becomes 'roster' (roster is a kind of list).
'i' inserted inside 'roster' is 'ROISTER'.
(Other definitions for roister that I've seen before include "Engage in party activity" , "Have noisy fun" , "riot" , "Carouse" , "I rest or (anag) - engage in noisy merrymaking" .)