One of those pieces of eight (5)
I believe the answer is:
octet
'eight?' is the definition.
('octet' can be a synonym of 'eight')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(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" .)