Slackened, lessened (6)
I believe the answer is:
abated
I believe this is a double definition.
'slackened' is the first definition.
(I've seen this in another clue)
'lessened' is the second definition.
(abating is a kind of lessening)
(Other definitions for abated that I've seen before include "grew smaller" , "Settled" , "Eased" , "Became more moderate" , "Relaxed" .)