Weaken instruction, say (6)
I believe the answer is:
lessen
'weaken' is the definition.
(I know that lessen is a more specific form of the action weaken)
'instruction say' is the wordplay.
'instruction' becomes 'lesson' (lesson is a kind of instruction).
'say' shows a homophone (sound like).
'lesson' is a homophone of 'LESSEN'.
(Other definitions for lessen that I've seen before include "Make fewer in number" , "Shrink" , "Become smaller or fewer" , "Decrease, diminish" , "Contract" .)