Essential, it's used by youth leader with energy (8)
I believe the answer is:
vitality
'energy' is the definition.
(I know that vitality is a type of energy)
'essential it's used by youth leader' is the wordplay.
'essential' becomes 'vital' (similar in meaning).
'used by' means one lot of letters go next to another.
'youth leader' becomes 'y' (I've seen this in other clues).
'vital'+'it'+'y'='VITALITY'
'with' acts as a link.
(Other definitions for vitality that I've seen before include "Vigour; ability to prosper" , "Liveliness" , "Vibrancy, energy" , "go" , "Physical energy" .)