For a girl to be in a new belt is quite justifiable (7)
I believe the answer is:
tenable
'justifiable' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for tenable that I've seen before include "Able to be held" , "Possible to be defended" , "Capable of being kept" , "Able to be defended against attack or objection" , "Believable" .)