That's a good one! (3)

I think it could be either:

Which length description are you looking for?:

ace

I believe this is a double definition.

'that's a good' is the first definition.
(thesaurus)

'one' is the second definition.
(ace has a value of one in some card games)

(Other definitions for ace that I've seen before include "Virtuoso" , "rare driving result?" , "Crack" , "In golf, a hole in one" , "Service that's never returned" .)

I've seen this clue in the Evening Standard.

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