Dry air circulating by delta (4)
I believe the answer is:
arid
'dry' is the definition.
(similar in meaning)
'air circulating by delta' is the wordplay.
'circulating' is an anagram indicator.
'by' means one lot of letters go next to another.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'air' is an anagram of 'ari'.
'ari'+'d'='ARID'
(Other definitions for arid that I've seen before include "Lacking sufficient rainfall" , "Tedious; short of rain" , "Dry and infertile" , "Parched with heat" , "Excessively dry" .)