One of those pieces of eight (5)

I believe the answer is:

octet

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'eight?' is the definition.
('octet' can be a synonym of 'eight')

I cannot understand how the rest of the clue works.

(Other definitions for octet that I've seen before include "Piece for eight players" , "Set of eight" , "Mendelssohn or Schubert piece" , "Work for eight players" , "Group of eight, say singers" .)

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

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