Rhizome, flower tip as well, rejected by reserve (9)
I believe the answer is:
rootstock
'rhizome' is the definition.
(I know that rhizome can be written as rootstock)
'flower tip as well rejected by reserve' is the wordplay.
'flower' becomes 'r' (abbreviation for river).
'tip' says the letters should be written backwards (I've seen 'tipped' mean this).
'as well' becomes 'too' (both can mean 'also').
'rejected by reserve' becomes 'stock' (I can't explain this - if you can you should believe this answer much more).
'too' in reverse letter order is 'oot'.
'r'+'oot'+'stock'='ROOTSTOCK'
Can you help me to learn more?
(Other definitions for rootstock that I've seen before include "Source" , "that extends underground" , "Rhizome" .)