Vapour from boiling water (5)
I believe the answer is:
steam
'vapour from boiling water' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for steam that I've seen before include "Pre-digital" , "Gas formed by vaporizing water at 100?C" , "Cook" , "Kind of power" , "Kettle emission" .)