Capacity to be happy (7)
I believe the answer is:
content
I believe this is a double definition.
'capacity' is the first definition.
('content' can be a synonym of 'capacity')
'happy' is the second definition.
('content' is associated in meaning with 'happy')
'to be' is the link.
(Other definitions for content that I've seen before include "Gist" , "Satisfied; volume" , "Satisfied - all within - capacity" , "Satisfied, happy enough" , "Satisfied - subject matter" .)