Enduring final gin cocktail (7)
I believe the answer is:
lasting
'enduring' is the definition.
('last' can be a synonym of 'endure')
'final gin cocktail' is the wordplay.
'final' becomes 'last' (synonyms).
'cocktail' is an anagram indicator.
'gin' with letters rearranged gives 'ing'.
'last'+'ing'='LASTING'
(Other definitions for lasting that I've seen before include "Enduring" , "long-term" , "there for long while" , "Evergreen" , "Persistent" .)