Samuel conceals Diana's humiliating cruelty (6)
I believe the answer is:
sadism
'humiliating cruelty' is the definition.
'sadism' can be an answer for 'cruelty' (deriving pleasure from cruelty). I'm unsure of the 'humiliating' bit.
'samuel conceals diana's' is the wordplay.
'samuel' becomes 'sam' (I can't justify this - if you can you should give a lot more credence to this answer).
'conceals' means one lot of letters goes inside another.
'diana's' becomes 'Di's' (Di is short for Diana).
'sam' enclosing 'dis' is 'SADISM'.
Can you help me to learn more?
(Other definitions for sadism that I've seen before include "Pleasure obtained by inflicting harm" , "Dim ass (anag)" , "Cruel behaviour" , "Taking pleasure in inflicting pain" , "Deliberate cruelty" .)