Gas discharge behind VA (6)
I believe the answer is:
vapour
'gas discharge behind va' is the definition.
I can't judge whether this defines the answer.
'discharge behind va' is the wordplay.
'discharge' becomes 'pour' (I can't explain this - if you can you should give a lot more credence to this answer).
'behind' is a charade indicator (letters next to each other).
'pour' put after 'va' is 'VAPOUR'.
Can you help me to learn more?
(Other definitions for vapour that I've seen before include "Boast" , "Moisture diffused and suspended in air" , "Liquid in gas form" , "Visible steam or clouds" , "Substance diffused in the air" .)