Instinctive person sure to succeed (7)
I believe the answer is:
natural
This clue is a double definition.
'instinctive' is the first definition.
(I know that instinctive can be written as natural)
'person sure to succeed' is the second definition.
(someone with a natural talent)
(Other definitions for natural that I've seen before include "Not artificial; off-white" , "Not made; off-white" , "pure" , "as one might expect" , "Normal; off-white" .)