Happy and endlessly at capacity (8)
I believe the answer is:
cheerful
'happy' is the definition.
(similar in meaning)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cheerful that I've seen before include "Lure chef into being in good spirits" , "Optimistic and upbeat" , "Showing good spirits" , "In good spirits, lighthearted" , "Bright" .)