She's not merely good, but superior (6)
I believe the answer is:
abbess
'superior' is the definition.
(abbess is a kind of superior)
'she's not merely good' is the wordplay.
'she' becomes 'Abbe'.
'not merely good' becomes 'ss' (I can't justify this - if you can you should believe this answer much more).
'abbe'+'ss'='ABBESS'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for abbess that I've seen before include "Senior ecclesiastic" , "Religious leader" , "Nuns' leader" , "woman trying to be religious?" , "Superior of convent" .)