Dry air circulating by day (4)
I believe the answer is:
arid
'dry' is the definition.
(similar in meaning)
'air circulating by day' is the wordplay.
'circulating' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'day' becomes 'd' (abbreviation for day).
'air' is an anagram of 'ari'.
'ari'+'d'='ARID'
(Other definitions for arid that I've seen before include "Excessively dry" , "Without much rainfall" , "Dry, devoid of interest" , "Very dry, having little rain" , "Dry and unfertile" .)