Fat lady in a boat? (5)

I believe the answer is:

eight

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

This is the entire clue.

Can you help me to learn more?

(Other definitions for eight that I've seen before include "Oxford crew, perhaps?" , "Have one over the . . . . . to be slightly drunk" , "Set of rowers" , "power of two" , "23" .)

I've seen this clue in the Evening Standard.

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