Nine others ordered imitation diamond (10)
I believe the answer is:
rhinestone
'imitation diamond' is the definition.
(I've seen this in another clue)
'nine others ordered' is the wordplay.
'ordered' is an anagram indicator.
'nine'+'others'='nineothers'
'nineothers' is an anagram of 'RHINESTONE'.
(Other definitions for rhinestone that I've seen before include "one not really brilliant?" , "Fake gem" , "imitation gem" , "Phoney" , "Rock" .)