Black treacle ruined jewellery (8)
I believe the answer is:
bracelet
'jewellery' is the definition.
(bracelet is a kind of jewellery)
'black treacle ruined' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'ruined' is an anagram indicator.
'treacle' with letters rearranged gives 'racelet'.
'b'+'racelet'='BRACELET'
(Other definitions for bracelet that I've seen before include "Bangle" , "Let caber be made into a piece of jewellery" , "Ornamental band worn on wrist" , "Rebel cat (anag.)" , "Ornament worn on arm or wrist" .)