One fighting for salvation? (4)

I believe the answer is:

army

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'one fighting for salvation?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

This is all the clue.

Can you help me to learn more?

(Other definitions for army that I've seen before include "Large military body" , "See 21" , "A defence force" , "Organisation for soldiering" , "Military land force" .)

I've seen this clue in the Evening Standard.

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