Strength, effort (6)
I believe the answer is:
energy
I believe this clue is a double definition.
'strength' is the first definition.
(I know that energy is a type of strength)
'effort' is the second definition.
(thesaurus)
(Other definitions for energy that I've seen before include "Go green, possibly with envy, ultimately" , "Power or vigorous activity" , "Exertion of force" , "Healthy capacity for vigorous activity" , "Zeal" .)