Dry, as a desert (4)
I believe the answer is:
arid
'dry as a desert' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arid that I've seen before include "Extremely dry" , "(Of land) very dry" , "Dry and unfertile" , "Parched and barren" , "Dry raid" .)