Declare money reserve (5)
I believe the answer is:
cache
'reserve' is the definition.
(I've seen this before)
'declare money' is the wordplay.
'declare' shows a homophone (sound like).
'money' becomes 'cash' (I've seen this before).
'cash' is a homophone of 'CACHE'.
(Other definitions for cache that I've seen before include "Conceal" , "Hidden store of things" , "Hiding place for treasure or store" , "Hiding place for treasure or stores" , "Something hidden" .)