The total amount of gravel (9)

I believe the answer is:

aggregate

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'the total amount' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for aggregate that I've seen before include "The whole amount" , "Solid rock" , "Get garage to do sum" , "Accumulation" , "Collected" .)

I've seen this clue in the Sydney Morning Herald.

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