Tell later about energy (6)
I believe the answer is:
relate
'tell' is the definition.
(relating is a kind of telling)
'later about energy' is the wordplay.
'about' indicates anagramming the letters.
'energy' becomes 'e' (physics symbol as in E=mc2**).
'later' anagrammed gives 'relat'.
'relat'+'e'='RELATE'
(Other definitions for relate that I've seen before include "Tell; a counselling service" , "Show a connection between (things)" , "Tell in Tralee" , "Give a report or show a connection" , "Recount" .)