Air out (9)
'air out' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for ventilate that I've seen before include "Let air enter and circulate" , "Allow fresh air to circulate" , "Superficial" , "Fan" , "Expose to a current of fresh air" .)
(Other definitions for deflate that I've seen before include "Let air out of, like balloon" , "Let the wind out of, braggart or balloon" , "Release gas from" , "Release air from" , "Take air" .)