Diminish a period of instruction, we hear (6)
I believe the answer is:
lessen
'diminish' is the definition.
(I know that diminish can be written as lessen)
'a period of instruction we hear' is the wordplay.
'a period of instruction' becomes 'lesson' (a lesson is instructional).
'we hear' indicates a 'sounds like' (homophone) clue.
'lesson' is a homophone of 'LESSEN'.
(Other definitions for lessen that I've seen before include "Make smaller for lenses" , "Reduce in number" , "Abate" , "Lower" , "Become smaller or fewer" .)