Justify; certificate (7)
I believe the answer is:
warrant
This clue is a double definition.
'justify' is the first definition.
(I know this)
'certificate' is the second definition.
(thesaurus)
(Other definitions for warrant that I've seen before include "Permit for arrest, search of premises, etc" , "Document giving authority" , "Guarantee" , "Guaranty - authorisation to make an arrest" , "Authorise, justify" .)