Banknotes reportedly in hidden store (5)
I believe the answer is:
cache
'hidden store' is the definition.
(I've seen this before)
'banknotes reportedly' is the wordplay.
'banknotes' becomes 'cash' (I've seen this in another clue).
'reportedly' shows a homophone (sound like).
'cash' sounds like 'CACHE'.
'in' acts as a link.
(Other definitions for cache that I've seen before include "Hiding place for treasure etc" , "Secret store of valuables" , "Hidden store of things" , "Something hidden" , "Collection of hidden things" .)