Energy put into singular achievement in games — like the Olympics (3-3)
I believe the answer is:
age-old
'like the olympics' is the definition.
(the Olympics are an ancient competition)
'energy put into singular achievement in games' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'put into' indicates putting letters inside.
'singular' becomes 'a' (both can mean one).
'achievement in games' becomes 'gold' (as in a gold medal in sports).
'a'+'gold'='agold'
'e' going into 'agold' is 'AGE-OLD'.
(Other definitions for age-old that I've seen before include "Hallowed by antiquity" , "Belonging to times long ago" , "Extremely ancient" , "Having existed for a very long time" , "Having been around for many years" .)