Diana's accused but let off (10)
I believe the answer is:
discharged
'let off' is the definition.
(I've seen this before)
'diana's accused' is the wordplay.
'diana's' becomes 'Di's' (Di is short for Diana).
'accused' becomes 'charged' ('charge' can be a synonym of 'accuse').
'dis'+'charged'='DISCHARGED'
'but' acts as a link.
(Other definitions for discharged that I've seen before include "released" , "Fired" , "let off" .)