Small case worn on a chain (6)
I believe the answer is:
locket
'small case worn on a chain' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for locket that I've seen before include "'Small piece of jewellery, it opens (6)'" , "Small ornamental case for photo etc." , "One round neck holding picture" , "Case on a neck chain" , "Small ornamental case worn on a chain" .)