Fresh air by day -- and dry! (4)
I believe the answer is:
arid
'dry' is the definition.
('arid' can be similar in meaning to 'dry')
'fresh air by day' is the wordplay.
'fresh' indicates an anagram.
'by' says to put letters next to each other.
'day' becomes 'd' (**).
'air' is an anagram of 'ari'.
'ari'+'d'='ARID'
'and' acts as a link.
(Other definitions for arid that I've seen before include "Tedious; short of rain" , "Dry raid" , "(Of climate) dry" , "Parched, waterless" , "flat" .)