Period of instruction is said to be moderate (6)
I believe the answer is:
lessen
'be moderate' is the definition.
(I've seen this in another clue)
'period of instruction is said' is the wordplay.
'period of instruction' becomes 'lesson' (a lesson is instructional).
'is said' shows a homophone (sound like).
'lesson' is a homophone of 'LESSEN'.
'to' is the link.
(Other definitions for lessen that I've seen before include "Shrink" , "Contract" , "Reduce in number" , "Make smaller or fewer" , "Make smaller for lenses" .)