Deposit of mineral aggregate beneath street (5)
I believe the answer is:
store
'deposit' is the definition.
(store is a kind of deposit)
'mineral aggregate beneath street' is the wordplay.
'mineral aggregate' becomes 'ore' (I've seen this before).
'beneath' says to put letters next to each other (in a down clue).
'street' becomes 'st'.
'ore' put after 'st' is 'STORE'.
'of' is the link.
(Other definitions for store that I've seen before include "Computer memory; supply kept for when needed" , "Depository for goods" , "Large shop; keep" , "Shop; hoard" , "Eg, granite" .)