Cruel behaviour is accepted by unhappy male (6)
I believe the answer is:
sadism
'cruel behaviour' is the definition.
(I've seen this before)
'is accepted by unhappy male' is the wordplay.
'accepted by' indicates putting letters inside (inserted letters are accepted).
'unhappy' becomes 'sad' (associated in meaning).
'male' becomes 'm' (common abbreviation).
'sad'+'m'='sadm'
'is' going into 'sadm' is 'SADISM'.
(Other definitions for sadism that I've seen before include "Perverse pleasure" , "Delight taken in cruelty" , "Deliberate cruelty" , "Taking pleasure in inflicting pain" , "Taking pleasure in cruelty" .)