Adult having energy; about time! (4)
I believe the answer is:
aeon
'time' is the definition.
(aeon is a kind of time)
'adult having energy about' is the wordplay.
'adult' becomes 'a' (abbreviation).
'having' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mcn).
'about' becomes 'on' (as in a book 'on' a certain topic).
'a'+'e'+'on'='AEON'
(Other definitions for aeon that I've seen before include "Epoch" , "long term" , "Indefinitely long period of time" , "Vast time period" , "A very long time, an age" .)