Flourishing growth of company that's hard to enter (9)
I believe the answer is:
bushiness
'flourishing growth' is the definition.
'bushiness' can be an answer for 'growth' (I have seen 'Expansive growth' mean 'bushiness' so perhaps 'growth' could also mean 'bushiness'). I'm unsure of the 'flourishing' bit.
'of company that's hard to enter' is the wordplay.
'of company' becomes 'business' (I can't justify this - if you can you should believe this answer much more).
'that's hard' becomes 'h' (abbreviation used in pencil classifications).
'to enter' means one lot of letters goes inside another.
'business' placed around 'h' is 'BUSHINESS'.
Can you help me to learn more?
(Another definition for bushiness that I've seen is " Expansive growth".)