Lessen, decrease (6)
I believe the answer is:
reduce
I believe this clue is a double definition.
'lessen' is the first definition.
(reducing is a kind of lessening)
'decrease' is the second definition.
(reducing is a kind of decreasing)
(Other definitions for reduce that I've seen before include "Make smaller or slimmer" , "Make smaller or take off weight" , "'Cut down, abridge (6)'" , "Make smaller or less" , "Lower" .)