Mob’s hidden supply, by the sound of it (5)
I believe the answer is:
horde
'mob's' is the definition.
(horde can mean a mob or crowd of people)
'hidden supply by the sound of it' is the wordplay.
'hidden supply' becomes 'hoard' (a hoard is a stock or supply of things).
'by the sound of it' shows a homophone (sound like).
'hoard' is a homophone of 'HORDE'.
(Other definitions for horde that I've seen before include "Many people" , "Great crowd" , "Large army" , "Large crowd" , "lots of people" .)