Famous lad with energy for exercising (9)
I believe the answer is:
legendary
'famous' is the definition.
(thesaurus)
I can't explain the rest of the clue.
(Other definitions for legendary that I've seen before include "Mythical - very famous" , "notorious" , "Relating to old or mythological tales" , "From a fabulous traditional story" , "Based on myth or famous for achievements (9)" .)