Clean horribly nasty air (8)
I believe the answer is:
sanitary
'clean' is the definition.
(I've seen this before)
'horribly nasty air' is the wordplay.
'horribly' indicates an anagram (a horribly wrong version of the letters).
'nasty'+'air'='nastyair'
'nastyair' with letters rearranged gives 'SANITARY'.
(Other definitions for sanitary that I've seen before include "Relating to health" , "Astray in hygienic way" , "Any sitar (anag.)" , "Hygienic and clean" , "Free from filth and pathogens" .)