Nunber you're up to when you're extremely busy (6)

I believe the answer is:

ninety

I'm a little stuck... Click here to teach me more about this clue!

'busy' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I can't see?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ninety that I've seen before include "old age" , "Quantity" , "Joe perhaps" , "fair amount" , "Four score and ten" .)

I've seen this clue in The Irish Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.