To begin with, lack of energy overwhelms wild fling's durability (4,4)
I believe the answer is:
long life
'durability' is the definition.
(ability to last a long time)
'to begin with lack of energy overwhelms wild fling's' is the wordplay.
'to begin with' suggests taking the first letters.
'overwhelms' means one lot of letters goes inside another.
'wild' indicates an anagram.
The first letters of 'lack of energy' is 'loe'.
'fling' anagrammed gives 'nglif'.
'loe' enclosing 'nglif' is 'LONG-LIFE'.
(Other definitions for long life that I've seen before include "A toast - to a prolonged existence" , "Type of milk" , "(Of milk etc) treated so as not to decay" , "treated to keep fresh" .)