Quite cunning to seal off vent (6)
I believe the answer is:
fairly
'quite' is the definition.
(thesaurus)
'cunning to seal off vent' is the wordplay.
'cunning' becomes 'fly' (fly can mean skilful or sharp).
'to seal off' means one lot of letters goes inside another.
'vent' becomes 'air' ('air' can be a synonym of 'vent').
'fly' placed around 'air' is 'FAIRLY'.
(Other definitions for fairly that I've seen before include "pretty" , "Rather" , "Evenhandedly" , "Moderately" , "To a moderate degree" .)