Imitation diamonds found on decapitated corpse (5)
I believe the answer is:
dummy
'imitation' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for dummy that I've seen before include "Mannequin" , "Feign to pass" , "Make as if to pass" , "Visible hand of cards" , "Unreal substitute" .)