Small case on a chain (6)
I believe the answer is:
locket
'small case on a chain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for locket that I've seen before include "Lucy" , "Small ornamental case hung round the neck" , "Small case on pendant, may hold photo" , "Ornamental case on necklace for memento" , "Case for carrying memento" .)