Venerable, old, brilliant and of ample proportions (10)

I believe the answer is:

voluminous

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'venerable' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?

I don't understand the rest of the clue.

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(Other definitions for voluminous that I've seen before include "roomy" , "Filling a lot of space" .)

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

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