Strongbox is unharmed (4)
I believe the answer is:
safe
This clue is a double definition.
'strongbox' is the first definition.
(I know that strongbox can be written as safe)
'unharmed' is the second definition.
(I've seen this in another clue)
'is' acts as a 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?" .)