One who secures the strongbox (6)
I believe the answer is:
locker
'the strongbox' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for locker that I've seen before include "Store for personal possessions" , "Small cupboard for personal use" , "Small securable cupboard" , "Box" , "Storage unit for personal possessions" .)