Band of light finally disappearing (4)
I believe the answer is:
Here is my best explanation:
'band' is the definition.
(I've seen this before)
'light finally disappearing' is the wordplay.
'finally' suggests the final letters.
'disappearing' becomes 'orc' (I am not sure about this - if you are sure you should believe this answer much more).
The last letter of 'light' is 't'.
'of' is the link.
(Other definitions for torc that I've seen before include "Metal neck ornament of ancient Britons", "Choker", "Necklace", "Ornament", "ancient necklace", "Ancient neck ornament", "Celtic metal neck-ring", "Rigid circular necklace", "Metallic neck ornament" and "Twisted metal neck band".)