Sort of dress said to restrain a disciplinarian? (9)

I believe the answer is:

strapless

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'sort of dress' is the definition.
(I know that strapless is a type of frock)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for strapless that I've seen before include "off one's shoulders" , "Daring dress is so" , "lacking support" , "As dress may be" .)

I've seen this clue in The Guardian.

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