Happy with things inside (7)
I believe the answer is:
content
'happy' is the definition.
(associated in meaning)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for content that I've seen before include "Satisfied - all within - capacity" , "substance of book?" , "Satisfied, happy enough" , "Quietly happy" , "Satisfied; volume" .)