A multitude in store, it’s said (5)
I believe the answer is:
horde
'a multitude' is the definition.
(horde is a kind of multitude)
'store it's said' is the wordplay.
'store' becomes 'hoard' (hoard is a kind of store).
'it's said' shows a homophone (sound like).
'hoard' sounds like 'HORDE'.
'in' acts as a link.
(Other definitions for horde that I've seen before include "A moving crowd" , "A swarm or multitude" , "Throng, mass" , "Large group of people" , "A mob of people" .)