Light fruit that contains extra energy for run (6)
I believe the answer is:
cheery
'light fruit that contains extra' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I can't see?
'energy for run' is the wordplay.
I cannot really see how this works, but
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is present in the answer.
'run' could be 'r' (cricket abbreviation) and 'r' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for cheery that I've seen before include "Joyful -- optimistic" , "Bright - pleasant - good-humoured" , "In good spirits" , "Happy, optimistic" , "Merry, jovial" .)