Investor being excellent is beginning to thrive (10)
I believe the answer is:
capitalist
'investor' is the definition.
(thesaurus)
'excellent is beginning to thrive' is the wordplay.
'excellent' becomes 'capital' (I've seen this before).
'beginning to' indicates taking the first letters.
The first letter of 'thrive' is 't'.
'capital'+'is'+'t'='CAPITALIST'
'being' acts as a link.
(Other definitions for capitalist that I've seen before include "investor" , "Free enterprise supporter" , "Seeker of profit in business" , "business person?" , "businessperson, maybe" .)