Some keener gymnasts go (6)
I believe the answer is:
energy
'go' is the definition.
(thesaurus)
'some keener gymnasts' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'ENERGY' is hidden within 'keener gymnasts'.
(Other definitions for energy that I've seen before include "Strength, vitality" , "Gusto" , "Go green, possibly with envy, ultimately" , "Power or vigorous activity" , "Dynamism" .)