Strongbox is unharmed (4)
I believe the answer is:
safe
I believe this is a double definition.
'strongbox' is the first definition.
(safe is a kind of strongbox)
'unharmed' is the second definition.
(I've seen this before)
'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?" .)