I trust mum discarded odd dancing costume? (4)

I believe the answer is:

tutu

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'costume?' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for tutu that I've seen before include "The old primate" , "Ballet-dancer's skirt" , "Ballet dancer's short skirt" , "outfit" , "Short dress" .)

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

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