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" .)
'official approval' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for imprimatur that I've seen before include "Church approval" , "church licence" .)