Flowering plant is decennial unusually (9)
I believe the answer is:
celandine
'flowering plant' is the definition.
(celandine is a kind of flowering plant)
'decennial unusually' is the wordplay.
'unusually' indicates anagramming the letters.
'decennial' with letters rearranged gives 'CELANDINE'.
'is' acts as a link.
(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" .)