Filter the air (6)
I believe the answer is:
strain
I believe this clue is a double definition.
'filter' is the first definition.
(I know that filter can be written as strain)
'the air' is the second definition.
('strain' can be a synonym of 'air')
(Other definitions for strain that I've seen before include "Injure (a muscle); distinct breed" , "In arts (anag)" , "Muscle injury, put liquid through sieve" , "Violent effort" , "Noted" .)