Strongroom — valuables are usually left there to begin with (5)
I believe the answer is:
vault
'strongroom' is the definition.
(vault is a kind of strongroom)
'valuables are usually left there to begin with' is the wordplay.
'to begin with' says to take the initial letters.
The first letters of 'valuables are usually left there' is 'VAULT'.
(Other definitions for vault that I've seen before include "Bank's strongroom" , "Crypt; leap" , "You can do this with a pole" , "Cellar; leap" , "bound?" .)