Sadly, legendary lad lost vitality (6)
I believe the answer is:
energy
'vitality' is the definition.
(I know that vitality can be written as energy)
'sadly legendary lad lost' is the wordplay.
'sadly' indicates an anagram.
'lost' indicates named letters should be taken away.
'legendary' with 'lad' taken out is 'egenry'.
'egenry' is an anagram of 'ENERGY'.
(Other definitions for energy that I've seen before include "Vigour, animation" , "Go green, possibly with envy, ultimately" , "Pep" , "Vigour, power" , "Power or vigorous activity" .)