Cooks over boiling water (6)
I believe the answer is:
steams
'cooks over boiling water' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for steams that I've seen before include "Rises as vapour" , "Cooks with water vapour" , "Emits vapours" , "Cooks using evaporated water" .)