Mercenary pair of Nationalists securing international agreement in Berlin (5)

I believe the answer is:

ninja

I'm a little stuck... Click here to teach me more about this clue!

'mercenary' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for ninja that I've seen before include "14th-century Japanese soldier of fortune" , "Feudal assassin" , "Stealthy Japanese fighter" , "One who steals" , "Stealthy warrior" .)

I've seen this clue in The Independent.

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