Guerrilla army close to sheik becoming huger after more arrived (5,5)

I believe the answer is:

khmer rouge

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'guerrilla' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see a link between them that I can't see?

I cannot understand how the remainder of the clue works.

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(Other definitions for khmer rouge that I've seen before include "Communist organisation" , "Cambodian terrorists" .)

I've seen this clue in the Sydney Morning Herald.

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