Block isolating Yankee prisoner? (6)

I believe the answer is:

stalag

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'block isolating yankee prisoner?' is the definition.
The definition suggests a singular noun which matches the answer.

This is the entire clue.

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(Other definitions for stalag that I've seen before include "German prison camp in WW2" , "Type of German prison camp" , "Water at the mouth" , "German prison_camp" , "German camp for POWs" .)

I've seen this clue in The Times and The Sunday Times.

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