Vast African desert (6)
I believe the answer is:
sahara
'vast african desert' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sahara that I've seen before include "World's largest sandy area" , "Hottest desert" , "Great African desert" , "About 3,500,000 square miles of Africa" , "Great desert in N Africa" .)