A vile tent - foul air (9)
I believe the answer is:
ventilate
'air' is the definition.
('ventilate' can be a synonym of 'air')
'a vile tent foul' is the wordplay.
'foul' indicates anagramming the letters.
'a'+'vile'+'tent'='aviletent'
'aviletent' is an anagram of 'VENTILATE'.
(Other definitions for ventilate that I've seen before include "Let air enter and circulate" , "Fan" , "Expose to a current of fresh air" , "Superficial" , "Allow fresh air to circulate" .)