Dry air circulating over delta (4)
I believe the answer is:
arid
'dry' is the definition.
(similar in meaning)
'air circulating over delta' is the wordplay.
'circulating' is an anagram indicator.
'over' means one lot of letters go next to another.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'air' anagrammed gives 'ari'.
'ari'+'d'='ARID'
(Other definitions for arid that I've seen before include "Dry and withered" , "Rainless (region)" , "Extremely dry" , "Very dry; uninteresting" , "Drought-hit" .)