Reinstate aggregate beneath base (7)
I believe the answer is:
restore
'reinstate' is the definition.
(synonyms)
'aggregate beneath base' is the wordplay.
'aggregate' becomes 'ore' (I've seen this before).
'beneath' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'base' becomes 'rest' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ore' after 'rest' is 'RESTORE'.
Can you help me to learn more?
(Other definitions for restore that I've seen before include "Return to original condition" , "Repair, renovate" , "Strengthen" , "Build up again" , "Bring back to original state" .)