Chest for valuables (6)
(Other definitions for casket that I've seen before include "Box for jewellery, or corpse" , "Box for jewellery or ashes" , "Ornamental box" , "Small box for valuables" , "Small box or chest" .)
'chest for valuables' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for coffer that I've seen before include "Money chest sounds like one with lung trouble" , "jewellery store?" , "Treasury - chest for valuables" , "safe" , "Chest for storing valuables" .)