Crossword Genius

A fat lady at bingo (5)

Ross

I believe the answer is:

eight

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

This might be a double definition.

'a fat lady at' is the first definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe they are linked in a way I don't understand?

'bingo' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

Can you help me to learn more?

(Other definitions for eight that I've seen before include "Cube root of five hundred and twelve" , "Carmina Burana composer" , "VIII" , "Several in a row" , "Oxygen's atomic number" .)

I've seen this clue in the Evening Standard.
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