A swell pair of letters (5)

I believe the answer is:

dandy

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'a swell' is the definition.
(I know that swell can be written as dandy)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for dandy that I've seen before include "Fine for American" , "See 6" , "Fop; excellent (US)" , "Fashionable fellow" , "one of stylish ways" .)

I've seen this clue in The Daily Express.

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