Deals including 100 pieces of fish (6)
I believe the answer is:
scales
'pieces of fish' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
I also know that 'fish' relates to this answer.
'deals including 100' is the wordplay.
'deals' becomes 'sales' (I can't explain this - if you can you should give a lot more credence to this answer).
'including' is an insertion indicator.
's00' becomes 'C' (Roman numeral).
'sales' going around 'c' is 'SCALES'.
Can you help me to learn more?
(Other definitions for scales that I've seen before include "Weighing devices, sign of Libra" , "Set of musical notes - balanced on a fish?" , "Removes plaque" , "Fish flakes or weighing instruments" , "Graduated sequence of musical notes" .)