Splendid act I completed, partially educational (8)
I believe the answer is:
didactic
'educational' is the definition.
(I've seen this before)
'splendid act i completed partially' is the wordplay.
'partially' says the answer is hidden in the clue.
'DIDACTIC' can be found hidden inside 'splendid act i completed'.
(Other definitions for didactic that I've seen before include "Having the manner of one lecturing students" , "In the manner of a (domineering) teacher" , "Teaching morally" , "Intended to teach or instruct" , "giving lesson" .)