Taken from vaults where entry's forbidden (3,2,6)
I believe the answer is:
out of bounds
'forbidden' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for out of bounds that I've seen before include "taboo" , "Off limits" , "like an exhausted kangaroo?" , "lying beyond field of play?" , "Skipper so exhausted" .)