Stout quality of bath is Ben's problem (9)
I believe the answer is:
tubbiness
'stout quality' is the definition.
The answer and definition can be both human attributes as well as being singular nouns.
Maybe there's a link between them I don't understand?
'bath is ben's problem' is the wordplay.
'bath' becomes 'tub' (synonyms).
'problem' indicates an anagram.
'is'+'bens'='isbens'
'isbens' with letters rearranged gives 'biness'.
'tub'+'biness'='TUBBINESS'
'of' is the link.
Can you help me to learn more?
(Another definition for tubbiness that I've seen is " needing to slim down from this state?".)