The case of a keepsake (6)
I believe the answer is:
locket
'the case' is the definition.
(locket is a kind of case)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for locket that I've seen before include "Small ornamental case-pendant" , "Case hung round the neck, containing a photo" , "Ornamental case on necklace for memento" , "Small ornamental case for photo etc." , "Lucy" .)