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 understand how one could define the other.

'wwi battleground nivelle evacuated with gas' is the wordplay.
'wwi battleground' becomes 'argo' (I can't justify this - if you can you should give a lot more credence to this answer).
'evacuated' suggests removing the centre (internal letters are evacuated from the word).
'with' says to put 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?