Almost mature enough for a formal dress (4)

I believe the answer is:

gown

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'formal dress' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for gown that I've seen before include "Long formal dress" , "Formal frock, hospital garb and ceremonial dress" , "Ball dress" , "Academic attire" , "Loose flowing garment for woman or judge" .)

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

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