Diminish, lessen (8)
I believe the answer is:
decrease
I believe this is a double definition.
'diminish' is the first definition.
(I know that diminish can be written as decrease)
'lessen' is the second definition.
(I know that lessen can be written as decrease)
(Other definitions for decrease that I've seen before include "Make or become less" , "Reduce, lessen" , "Grow smaller or less" , "Turn down" , "Make smaller or fewer" .)