Like fizzless soda (4)
I believe the answer is:
flat
'like fizzless soda' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for flat that I've seen before include "one or more in uniform" , "razed to the ground, having had blow-out" , "Apartment - below the note" , "character on the staff" , "Quarters - punctured - smooth" .)