Cut down, lessen (6)
I believe the answer is:
reduce
I believe this is a double definition.
'cut down' is the first definition.
(synonyms)
'lessen' is the second definition.
(reducing is a kind of lessening)
(Other definitions for reduce that I've seen before include "Decrease" , "Cut down on, lessen" , "subdue" , "Make smaller or slimmer" , "Cut down on,lessen" .)