Like a sauna (5)
'like a sauna' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for steamy that I've seen before include "Erotic; like vapour" , "risque behaviour?" , "Vaporous: lustful" , "Passionate" , "Erotic, sexy" .)
(Other definitions for humid that I've seen before include "Sultry" , "Rather wet" , "Muggy" , "Describes air that contains a great deal of water vapour" , "Dank" .)