Clear air, initially cold, above desert (6)
I believe the answer is:
acquit
'clear' is the definition.
(I know that clear can be written as acquit)
'air initially cold above desert' is the wordplay.
'initially' suggests taking the first letters.
'cold' becomes 'c' (eg on taps).
'above' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'desert' becomes 'quit' (I've seen this before).
The initial letter of 'air' is 'a'.
'a'+'c'+'quit'='ACQUIT'
(Other definitions for acquit that I've seen before include "Declare to be innocent in court" , "Discharge as innocent, dismiss charge" , "Exculpate" , "Absolve" , "Let off" .)