Strongbox is protected (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)
'is protected' is the second definition.
(similar in meaning)
(Other definitions for safe that I've seen before include "Free from risk" , "Unharmed" , "Reliable" , "Metal container for valuables" , "Out of harm's way" .)