Man with libertine cavorting without end (12)
I believe the answer is:
interminable
'without end' is the definition.
(thesaurus)
'man with libertine cavorting' is the wordplay.
'with' says to put letters next to each other.
'cavorting' indicates anagramming the letters.
'man' put after 'libertine' is 'libertineman'.
'libertineman' is an anagram of 'INTERMINABLE'.
(Other definitions for interminable that I've seen before include "Without end" , "Endless" , "Tedious" , "Long drawn-out" , "Seeming likely to go on for ever" .)