Make less serious (8)
(Other definitions for mitigate that I've seen before include "Appease" , "Moderate" , "Lessen the seriousness of" , "Lessen the gravity of" , "Make less harsh" .)
(Other definitions for extenuate that I've seen before include "Mitigate" , "Lessen the seriousness of (an offence) with a mitigating factor" .)