Too young, really, to be a fat lady (5)

I believe the answer is:

eight

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

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for eight that I've seen before include "No. of stones in cwt." , "Mathematical cube" , "Team involved in row" , "4x2" , "VIII" .)

I've seen this clue in the Evening Standard.

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