Casting, otherwise containing metal block (5)
I believe the answer is:
ingot
'metal block' is the definition.
(I've seen this in another clue)
'casting otherwise containing' is the wordplay.
'containing' indicates the answer is hidden within the clue.
'INGOT' is hidden in the letters of 'casting otherwise'.
(Other definitions for ingot that I've seen before include "A lump of metal, perhaps gold" , "Metallic bar" , "Metal casting" , "Bar of bullion, for example" , "Piece of cast metal" .)