No worry with energy at a minimum in early period (6)
I believe the answer is:
nonage
'early period' is the definition.
(term for someone's youth)
'no worry with energy at a minimum' is the wordplay.
'worry' becomes 'nag' (nagging is a kind of worrying).
'with' is a charade indicator (letters next to each other).
'at a minimum' indicates taking the first letters.
The initial letter of 'energy' is 'e'.
'no'+'nag'+'e'='NONAGE'
'in' is the link.
(Other definitions for nonage that I've seen before include "minority as before" , "In law, state of being under age for certain things" , "State of being unable to marry" , "Period of immaturity" , "(Legal) infancy" .)