Excessively instructive (8)
I believe the answer is:
didactic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">excessively instructive</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for didactic that I've seen before include "Instructive, perhaps overly so" , "like a school-teacher?" , "In the manner of a (domineering) teacher" , "Having the manner of one instructing pupils" , "(Patronisingly) educational" .)