System for weighing Helen's home (4)

I believe the answer is:

troy

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'helen's home' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

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(Other definitions for troy that I've seen before include "City besieged to releae Helen" , "Ancient city; type of weight" , "Ancient city of the Iliad" , "Precious stone weight system" , "The war for it, Homer" .)

I've seen this clue in the Sydney Morning Herald.

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