Ballet group in Berlin prison (7)

I believe the answer is:

spandau

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'berlin prison' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for spandau that I've seen before include "cage in Berlin" , "British band, -- Ballet" , "Berlin prison demolished in 1987 after the death of its last prisoner, Rudolf Hess" , "Rudolf Hess's prison" , "-- Ballet, pop group" .)

I've seen this clue in The Guardian.

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