In right environment seedling's beginning to flourish (7)
I believe the answer is:
prosper
'to flourish' is the definition.
('prosper' can be a synonym of 'flourish')
'in right environment seedling's' is the wordplay.
'in' indicates putting letters inside.
'right' becomes 'proper' (associated in meaning).
'environment seedling' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'proper' going around 's' is 'PROSPER'.
'beginning' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for prosper that I've seen before include "Do well, be successful in life" , "Succeed, flourish" , "Flourish, do well" , "Get rich" , "Make good" .)