Born again Hitlerite, but he’s mostly just gas (3-4)
I believe the answer is:
neo-nazi
'he's mostly just gas' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'born again hitlerite' is the wordplay.
'born again' becomes 'neo' (I can't explain this - if you can you should give a lot more credence to this answer).
'hitlerite' becomes 'nazi' (I've seen this before).
'neo'+'nazi'='NEO-NAZI'
'but' is the link.
Can you help me to learn more?
(Other definitions for neo-nazi that I've seen before include "Modern disciple of Hitler" , "Extreme nationalist" , "Modern Hitlerite" .)