Hot and humid (6)
I believe the answer is:
'hot and humid' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sultry that I've seen before include "Such weather is hot and oppressive" , "Such weather is close and oppressive" , "Passionate" , "Sweltering - arousing passion" , "Close -- sensual" .)