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