Virginia to decant gas (6)
I believe the answer is:
'gas' is the definition.
(I know that vapour is a type of gas)
'virginia to decant' is the wordplay.
'virginia' becomes 'va' (US state abbreviation).
'to decant' becomes 'pour' ('pour' can be a synonym of 'decant').
(Other definitions for vapour that I've seen before include "Moisture diffused and suspended in air" , "Particles suspended in air" , "Substance in the form of a mist, fume or smoke" , "Airborne condensation" , "Liquid in gas form" .)