Excessively instructive (8)
I believe the answer is:
didactic
'excessively instructive' is the definition.
(I've seen this before)
This is all the clue.
(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" .)