Dion also into yellow-flowering plant (9)
I believe the answer is:
celandine
'flowering plant' is the definition.
(celandine is a kind of flowering plant)
'dion also into' is the wordplay.
'dion' becomes 'celine' (I can't explain this - if you can you should give a lot more credence to this answer).
'also' becomes 'and' (both can mean in addition).
'into' indicates putting letters inside.
'celine' placed around 'and' is 'CELANDINE'.
'yellow' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for celandine that I've seen before include "Land niece with the swallowwort." , "Flowering plant - decennial (anag)" , "... one blooming" , "Plant of buttercup family" .)