Scoundrel and libertine, good at heart (5)
I believe the answer is:
rogue
'scoundrel' is the definition.
(rogue is a kind of scoundrel)
'libertine good at heart' is the wordplay.
'libertine' becomes 'roue' (roue is a kind of libertine).
'good' becomes 'g' (abbreviation).
'at heart' is an insertion indicator.
'roue' placed around 'g' is 'ROGUE'.
'and' acts as a link.
(Other definitions for rogue that I've seen before include "Scamp" , "Mischievous person" , "Impish, likeable person" , "Baddy" , "Dishonest man" .)