Small box (6)
'small box' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for casket that I've seen before include "Jewel box" , "Small box to hold jewels" , "Small box or chest" , "Helping boost" , "Small box - coffin" .)
(Other definitions for carton that I've seen before include "Cantor (anag)" , "He did a far, far better thing" , "fictional lawyer" , "Light cardboard box" , "Small, light box or container" .)