First lady to occupy chair, unusually successful person (8)
I believe the answer is:
achiever
'successful person' is the definition.
(someone who has achieved things)
'first lady to occupy chair unusually' is the wordplay.
'first lady' becomes 'eve' (first woman in the Bible).
'to occupy' is an insertion indicator (I've seen this in other clues).
'unusually' indicates anagramming the letters.
'chair' anagrammed gives 'achir'.
'eve' put inside 'achir' is 'ACHIEVER'.
(Other definitions for achiever that I've seen before include "success?" , "Successful person" , "high-flier" , "I successfully conclude" , "One who succeeds" .)