Lessen, subside (5)
I believe the answer is:
abate
I believe this clue is a double definition.
'lessen' is the first definition.
(abating is a kind of lessening)
'subside' is the second definition.
(I've seen this before)
(Other definitions for abate that I've seen before include "Lessen in intensity" , "Relax" , "Lessen - cease" , "Subside, grow less" , "Lose intensity" .)