Quite clever, letting the air in (6)
I believe the answer is:
fairly
'quite' is the definition.
(I've seen this before)
'clever letting the air in' is the wordplay.
I cannot really see how this works, but
'the air' could be 'air' and 'air' is found in the answer.
'clever' could be 'fly' and 'fly' is located in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for fairly that I've seen before include "Evenhandedly" , "Rather" , "To a moderate degree" , "In an evenhanded manner - somewhat" , "pretty" .)