Capacity for happiness (7)
I believe the answer is:
content
I believe this clue is a double definition.
'capacity' is the first definition.
(I know that capacity can be written as content)
'for happiness' is the second definition.
(thesaurus)
(Other definitions for content that I've seen before include "Gist" , "Satisfied - subject matter" , "Elements" , "Gratify" , "Satisfied - all within - capacity" .)