Grub stored in cellar vaults (5)
I believe the answer is:
larva
'grub' is the definition.
(I know that grub can be written as larva)
'stored in cellar vaults' is the wordplay.
'stored in' indicates the answer is hidden within the clue.
'LARVA' can be found hidden inside 'cellar vaults'.
(Other definitions for larva that I've seen before include "Immature form of caterpillar" , "Early stage of creature" , "Immature life form" , "Juvenile form of insect" , "Volcanic outpouring" .)