Strong male entering ninth decade perhaps losing energy (6)
I believe the answer is:
mighty
'strong' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for mighty that I've seen before include "Extremely" , "Strong, powerful" , "Powerful - exalted" , "Powerful - huge" , "Having great strength" .)