Copy poster principally offering cheap diamond (9)
I believe the answer is:
carbonado
'cheap diamond' is the definition.
'copy poster principally offering' is the wordplay.
'copy' becomes 'carbon' (carbon is a kind of copy**).
'poster' becomes 'ad'.
'principally' says to take the initial letters.
The initial letter of 'offering' is 'o'.
'carbon'+'ad'+'o'='CARBONADO'
(Other definitions for carbonado that I've seen before include "Beef stew; black diamond" , "Dark diamond used in industry for drilling and polishing" , "Meat grilled" .)