Selling plants is a rapidly developing business (6,8)
I believe the answer is:
growth industry
'selling plants is a rapidly' is the definition.
I can't tell whether this defines the answer.
'developing business' is the wordplay.
'developing' becomes 'growth' (I can't explain this - if you can you should give a lot more credence to this answer).
'business' becomes 'industry' (industry is a kind of business).
'growth'+'industry'='GROWTH INDUSTRY'
Can you help me to learn more?
(Another definition for growth industry that I've seen is " Business that is developing and expanding".)