Our navy goes inside a border for battle (6)

I believe the answer is:

arnhem

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

I cannot understand how the remainder of the clue works.

(Other definitions for arnhem that I've seen before include "Second World War battle" , "WW2 battle" , "Dutch city" , "Netherlands scene of 1944 battle" , "Site of 1944 battle" .)

I've seen this clue in The Sun.

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