Reserve supply of sound money (5)
I believe the answer is:
cache
'reserve supply' is the definition.
(thesaurus)
'sound money' is the wordplay.
'sound' shows a homophone (sound like).
'money' becomes 'cash' (cash is a type of money).
'cash' sounds like 'CACHE'.
'of' is the link.
(Other definitions for cache that I've seen before include "(Place in) hidden store" , "Hidden store of things" , "Secret store, hoard" , "what things could be stored in" , "Concealed store" .)