A secret store of sound money (5)
I believe the answer is:
cache
'a secret store' is the definition.
(I've seen this before)
'sound money' is the wordplay.
'sound' shows a homophone (sound like).
'money' becomes 'cash' (cash is a type of money).
'cash' is a homophone of 'CACHE'.
'of' is the link.
(Other definitions for cache that I've seen before include "Stockpile" , "Hidden storage place" , "Nest egg" , "Reserve" , "what things could be stored in" .)