Cook, as bao buns (5)
I believe the answer is:
steam
'cook as bao buns' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for steam that I've seen before include "Old-fashioned power" , "Watery vapour" , "Invisible gas from boiling water" , "Vaporous exhalation" , "comes with a hiss?" .)