A baozi is one (3)
I believe the answer is:
bun
'a baozi is one' is the definition.
(I know that baozi is a type of bun)
This is the entire clue.
(Other definitions for bun that I've seen before include "Small rounded bread or cake" , "One from Chelsea" , "Small cake; hair style" , "Kind of small round cake" , "Small cake - tied-up hair" .)