Describes air that contains a great deal of water vapour (5)
I believe the answer is:
humid
'describes air that contains a great deal of water vapour' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for humid that I've seen before include "Describes air that contains a great deal of water vapour" , "Dank" , "(Of atmosphere) with high water vapour content" , "Moist, warm (weather)" , "Close" .)