Tributary of the Narew River forming part of Poland's border with both Belarus and Ukraine (3)

I believe the answer is:

bug

(Other definitions for bug that I've seen before include "Irritate" , "Insect, or flaw in computer program" , "Insect - virus" , "Hidden microphone" , "Small crawling creature, now in computer programs" .)

I've seen this clue in The Mirror.

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