Strength is concealed by keener gymnasts (6)
I believe the answer is:
energy
'strength' is the definition.
(I know that energy is a type of strength)
'concealed by keener gymnasts' is the wordplay.
'concealed by' indicates a hidden word.
'ENERGY' is hidden in the letters of 'keener gymnasts'.
'is' acts as a link.
(Other definitions for energy that I've seen before include "'Vigour, vitality (6)'" , "Vigour, spirit" , "Power or vigorous activity" , "Capacity for action" , "Pep" .)