Proper moisture on the air (3)
I believe the answer is:
due
'proper' is the definition.
(what is due is what is proper or appropriate)
'moisture on the air' is the wordplay.
'moisture' becomes 'dew' (I have seen 'Overnight moisture ' mean 'dew' so perhaps 'moisture' could also mean 'dew').
'on the air' shows a homophone (sound like).
'dew' is a homophone of 'DUE'.
(Other definitions for due that I've seen before include "Scheduled to arrive" , "deserts" , "Expected; appropriate" , "Deserved or unpaid" , "Ripe for payment or arrival" .)