Pitiless, slippery character lives in a pigpen (6)
I believe the answer is:
steely
'pitiless' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?
'slippery character lives in a pigpen' is the wordplay.
'slippery character' becomes 'eel' (eel can mean a slippery person).
'lives in' indicates putting letters inside.
'a pigpen' becomes 'sty' (synonyms).
'eel' going into 'sty' is 'STEELY'.
Can you help me to learn more?
(Other definitions for steely that I've seen before include "Coldly determined" , "Ruthless" , "Gimlet-eyed" , "With (cold) resolve" , "Inflexibly severe" .)