A tidy sum is a considerably larger amount of money than expected. It fetched a tidy sum.
The tidy in a tidy sum means substantial rather than neat, which is an old use. To describe money as a tidy sum is British understatement doing its usual work.
1phrasea tidy sumA considerably larger amount of money than expected.
“The old clock went for a tidy sum at auction.”
a tidy sum is a phrase.
Practised in Idioms