Abate, reduce (6)
I believe the answer is:
lessen
I believe this is a double definition.
'abate' is the first definition.
(I know that abate is a more specific form of lessen)
'reduce' is the second definition.
(I know that reduce is a more specific form of lessen)
(Other definitions for lessen that I've seen before include "Shrink" , "Decrease, diminish" , "decline" , "Contract" , "Become smaller or fewer" .)