Land Army changing sides to begin (6)

I believe the answer is:

region

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'land' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for region that I've seen before include "Area,territory" , "Area having definable character" , "Tract or area" , "Ignore (anag) -- expanse of land" , "Part of a country" .)

I've seen this clue in The Telegraph.

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