Money, by the sound of it, in a hidden store (5)
I believe the answer is:
cache
'a hidden store' is the definition.
(I've seen this in another clue)
'money by the sound of it' is the wordplay.
'money' becomes 'cash' (cash is a type of money).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'cash' sounds like 'CACHE'.
'in' is the link.
(Other definitions for cache that I've seen before include "Hiding place for treasure or store" , "Secret source" , "Nest egg" , "Stockpile" , "Put away" .)