Inferior brought back tied up when Conservatory finally ruled out (3)

I believe the answer is:

sub

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'inferior' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

(Other definitions for sub that I've seen before include "Replacement" , "Underwater vessel" , "Advance payment" , "Beneath on bus" , "Loan" .)

I've seen this clue in The Independent.

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