Cruel person, miserable, is repentant ultimately (6)
I believe the answer is:
sadist
'cruel person' is the definition.
(someone who derives pleasure from inflicting pain)
'miserable is repentant ultimately' is the wordplay.
'miserable' becomes 'sad' (similar in meaning).
'ultimately' suggests the final letters.
The final letter of 'repentant' is 't'.
'sad'+'is'+'t'='SADIST'
(Other definitions for sadist that I've seen before include "One who delights in cruelty" , "One taking pleasure in cruelty" , "An amalgam, made up of two or more parts" , "One enjoying inflicting pain" , "One who takes delight in cruelty" .)