Business area with people beginning to thrive. Heavens! (9)
I believe the answer is:
firmament
'heavens' is the definition.
(synonyms)
'business area with people beginning to thrive' is the wordplay.
'business' becomes 'firm' (firm is a kind of business).
'area' becomes 'a' (maths abbreviation).
'with' is a charade indicator (letters next to each other).
'people' becomes 'men' (men are people).
'beginning to' suggests taking the first letters.
The first letter of 'thrive' is 't'.
'firm'+'a'+'men'+'t'='FIRMAMENT'
(Other definitions for firmament that I've seen before include "The starry heaven" , "Sky (literary)" , "Heavens, sky" , "The celestial sphere" , "outer space, encapsulated" .)