Period of instruction (6)
I believe the answer is:
lesson
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">period of instruction</span>' is the definition.<br/ ><span class="explanation-format__explanation">(a lesson is instructional)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for lesson that I've seen before include "Unit of instruction" , "Period of learning" , "Spell of teaching" , "Reading or exercise to be studied" , "See 1" .)