Make less severe (9)
'make less severe' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mitigate that I've seen before include "Extenuate" , "Appease" , "Lessen the seriousness of" , "Make less harsh" , "Assemble" .)
(Other definitions for relax that I've seen before include "Loosen up, slow down" , "Slow down and unwind" , "Make (rule) less strict" , "Take one's ease" , "Take it easy, don't be so tense" .)
(Other definitions for alleviate that I've seen before include "Relieve" , "Lessen strength of" , "Make less severe, say suffering" , "Make (pain) less intense" , "Make light" .)