Branch installing new vault (6)

I believe the answer is:

spring

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'vault' is the definition.
(thesaurus)

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

(Other definitions for spring that I've seen before include "The bouncy season" , "Bound; source" , "Flowering season of 7 dn" , "Leap - part of year" , "past February is it?" .)

I've seen this clue in The Times and The Sun.

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