One who prefers late hours with long parties (5,3)
I believe the answer is:
night owl
'one who prefers late hours' is the definition.
(night owl can mean someone who stays up late)
'with long parties' is the wordplay.
'parties' indicates an anagram (the letters move or party around).
'with'+'long'='withlong'
'withlong' with letters rearranged gives 'NIGHT-OWL'.
(Other definitions for night owl that I've seen before include "Person active in the small hours" , "Person who likes to be active late in the day" , "One likes to be active late into the evening" , "he may go to bed at last" , "One working late" .)