Official approval (6)
(Other definitions for assent that I've seen before include "Official sanction" , "Agreement (to a proposal)" , "Consent or approval" , "Give consent" , "Agree, acquiesce" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">official approval</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for imprimatur that I've seen before include "Church approval" , "church licence" .)