Artillerymen called back by old king for final battle (8)

'final battle' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'artillerymen called back by old king' is the wordplay.
'artillerymen' becomes 'ra' (Royal Artillery).
'called back' is a reversal indicator.
'by old' becomes 'orang' (I can't justify this - if you can you should give a lot more credence to this answer).
'king' becomes 'K' (abbreviation for king in chess).
'orang' back-to-front is 'gnaro'.
'ra'+'gnaro'+'k'='RAGNAROK'

'for' is the link.

Can you help me to learn more?