Feeling sorrow is married to cruelty (6)
I believe the answer is:
sadism
'cruelty' is the definition.
(deriving pleasure from cruelty)
'feeling sorrow is married' is the wordplay.
'feeling sorrow' becomes 'sad' (I can't justify this - if you can you should believe this answer much more).
'married' becomes 'm'.
'sad'+'is'+'m'='SADISM'
'to' is the link.
Can you help me to learn more?
(Other definitions for sadism that I've seen before include "Deliberate cruelty" , "Enjoyment taken in inflicting pain or cruelty" , "Cruel behaviour" , "Delight taken in cruelty" , "Pleasure obtained by inflicting harm" .)