Happy with what's inside? (7)
I believe the answer is:
content
This clue is a double definition.
'happy' is the first definition.
(associated in meaning)
'what's inside' is the second definition.
(the contents of something)
'with' acts as a link.
(Other definitions for content that I've seen before include "Gratify" , "Satisfied - all within - capacity" , "what's included" , "Quietly happy" , "Elements" .)