Circular band nearly full of golden petals (7)
I believe the answer is:
corolla
'petals' is the definition.
(I have seen 'Petals of flower' mean 'corolla' so perhaps 'petals' could also mean 'corolla')
'circular band nearly full of golden' is the wordplay.
'circular band' becomes 'collar' (a collar is a band of material).
'nearly' means to remove the last letter.
'full of' means one lot of letters goes inside another.
'golden' becomes 'or' ('or' is the name for gold in heraldry).
'collar' with its last letter taken away is 'colla'.
'colla' enclosing 'or' is 'COROLLA'.
(Other definitions for corolla that I've seen before include "Whorl of flower petals" , "Internal whorl of a flower" , "bit of a bloomer" , "Flower's petals collectively" , "Part of a flower" .)