Franc note in money box (6)
I believe the answer is:
coffin
'box' is the definition.
(coffin is a kind of box)
'franc note in money' is the wordplay.
'franc' becomes 'f' (abbreviation for franc).
'note' becomes 'f' (musical note).
'in' indicates putting letters inside.
'money' becomes 'coin' (I've seen this before).
'f'+'f'='ff'
'ff' going inside 'coin' is 'COFFIN'.
(Other definitions for coffin that I've seen before include "Box for the dead" , "Corpse's chest" , "Box for a corpse" , "Casket for the dead" , "Sarcophagus" .)