Justify; certificate (7)
I believe the answer is:
warrant
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">justify</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know this)</span><br/ ><br/ >'<span class="explanation-format__original">certificate</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ ></span>
(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" .)