Someone sure to succeed but isn't sharp (7)
I believe the answer is:
natural
I believe this is a double definition.
'someone sure to succeed' is the first definition.
(someone who is a natural at something)
'isn't sharp' is the second definition.
(musical sharps and flats are not natural)
'but' is the link.
(Other definitions for natural that I've seen before include "Ordinary" , "Everyday" , "as one might expect" , "Unaffected; unaltered" , "pure" .)