Strongbox is unharmed (4)
I believe the answer is:
safe
This clue is a double definition.
'strongbox' is the first definition.
(I know that safe is a type of strongbox)
'unharmed' is the second definition.
(I've seen this in another clue)
'is' is the link.
(Other definitions for safe that I've seen before include "Iron box in which to keep money or important papers" , "Unharmed" , "Metal case for storing valuables" , "Secure - place to keep cash" , "Container that may be cracked?" .)