WWI battleground Nivelle evacuated with gas coming over (7)

'over' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'wwi battleground nivelle evacuated with gas' is the wordplay.
'wwi battleground' becomes 'argo' (I can't explain this - if you can you should give a lot more credence to this answer).
'evacuated' means to remove the middle letters (internal letters are evacuated from the word).
'with' is a charade indicator (letters next to each other).
'gas' becomes 'N' (chemical symbol for nitrogen).
'nivelle' with its middle removed is 'ne'.
'ne' put after 'n' is 'nne'.
'argo'+'nne'='ARGONNE'

'coming' is the link.

Can you help me to learn more?