Dud principal riding swell (4,7)

I believe the answer is:

lead balloon

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'dud' is the definition.
I know nothing about this answer so I can't judge whether this works.

'principal riding swell' is the wordplay.
'principal' becomes 'lead' (synonyms).
'riding' is a charade indicator (letters next to each other) (in a down clue, some letters go on top of others).
'swell' becomes 'balloon' (both can mean to expand or increase).
'lead'+'balloon'='LEAD BALLOON'

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I've seen this clue in the Sydney Morning Herald.

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