Case for a sword or knife (6)
I believe the answer is:
sheath
This clue is a double definition.
'case for a sword' is the first definition.
(I've seen this before)
'knife' is the second definition.
(sheath knife is a kind of knife)
(Other definitions for sheath that I've seen before include "Protective covering for a knife" , "cocktail dress" , "Blade-cover" , "Condom" , "case for weapon" .)