Ornamental breastplate (8)
I believe the answer is:
pectoral
'ornamental breastplate' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pectoral that I've seen before include "Of the chest or breast, like muscles" , "Alert cop about chest" , "Ornamental breastplate" , "Relating to the breast or chest" , "Of the chest, say muscles" .)