In 'Macbeth', a debt collector with a metal box (6)
I believe the answer is:
duncan
'in macbeth' is the definition.
The definition suggests a singular noun which matches the answer.
'a debt collector with a metal box' is the wordplay.
'a debt collector' becomes 'dun' (historical term for a debt collector).
'with' says to put letters next to each other.
'a metal box' becomes 'can' (I can't justify this - if you can you should believe this answer much more).
'dun'+'can'='DUNCAN'
Can you help me to learn more?
(Other definitions for duncan that I've seen before include "innovative dancer" , "King murdered by ambitious Macbeth" , "Man's name" , "Isadora ..., pioneer of modern dance" , "Scottish king" .)