Water vapour (5)
I believe the answer is:
steam
'water vapour' is the definition.
(steam from boiling water)
This is the entire clue.
(Other definitions for steam that I've seen before include "Kettle emission" , "Source of energy" , "comes with a hiss?" , "Gas formed by vaporizing water at 100?C" , "Energy (colloq.)" .)