Fumes (5)
'fumes' is the definition.
('reek' can be a synonym of 'fume')
This is the entire clue.
(Other definitions for reeks that I've seen before include "Smells very strongly" , "Stinks to high heaven" , "Smells bad" , "Gives off a strong unpleasant smell" , "Macgillycuddy's - -" .)
(Other definitions for vapour that I've seen before include "Visible steam or clouds" , "Air-like substance created by heating, like gas" , "Particles suspended in air" , "Boast" , "Substance in the form of a mist, fume or smoke" .)