Garment on friend covering one part of limb (8)

I believe the answer is:

brachium

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'limb' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'garment on friend covering one part of' is the wordplay.
'garment' becomes 'bra' (bra is a type of garment).
'on friend covering' becomes 'chum' (I am not sure about this - if you are sure you should believe this answer much more).
'one' becomes 'i' (Roman numeral).
'part of' means one lot of letters goes inside another.
'chum' enclosing 'i' is 'chium'.
'bra'+'chium'='BRACHIUM'

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I've seen this clue in The Telegraph.

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