It's normally measured at arm's length (6)

I believe the answer is:

sleeve

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'arm's length' is the definition.
I know that 'arm' relates to this answer. I also know that both the answer and definition are singular nouns

I don't understand how the rest of the clue works.

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(Other definitions for sleeve that I've seen before include "Cover for a gramophone record" , "Covering for arm or record" , "Covering for arm or sleeve" , "Some wear their heart on it" , "Some wear their hearts on it" .)

I've seen this clue in The Globe and Mail.

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