Stale wind sends American packing (4)
I believe the answer is:
flat
'stale' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for flat that I've seen before include "Not hilly" , "Prostrate" , "Dull; apartment" , "Not sloping; apartment" , "Absolutely" .)