Dress that’s loose at the back placed in metal container (6)
I believe the answer is:
caftan
'dress that's loose' is the definition.
'the back placed in metal container' is the wordplay.
'the back' becomes 'aft' (I've seen this before).
'placed in' is an insertion indicator (I've seen this in other clues).
'metal container' becomes 'can' (I've seen this before).
'aft' put within 'can' is 'CAFTAN'.
'at' acts as a link.
(Other definitions for caftan that I've seen before include "Loose tunic" , "Cotton or silk ankle-length garment with full sleeves worn by men in the Levant" , "Long cloak-like garment with sleeves" , "Type of long dress with wide sleeves" , "Long loose garment" .)