One avoids late night parties finally (2,6)
I believe the answer is:
at length
'finally' is the definition.
(synonyms)
'one avoids late night parties' is the wordplay.
'one' becomes 'i' (roman numeral).
'avoids' is a deletion indicator.
'parties' indicates anagramming the letters (the letters move or party around).
'late'+'night'='latenight'
'latenight' with 'i' taken away is 'latenght'.
'latenght' with letters rearranged gives 'AT LENGTH'.
(Other definitions for at length that I've seen before include "In exhaustive detail, after a long time" , "In detail, taking a long time" , "In full detail" , "Finally" , "In detail, fully" .)