Sponsor about to secure one plant (8)
I believe the answer is:
angelica
'plant' is the definition.
(angelica is a kind of plant)
'sponsor about to secure one' is the wordplay.
'sponsor' becomes 'angel' (angel is a kind of sponsor).
'about' becomes 'ca' (abbreviation for 'circa'**).
'to secure' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'angel'+'ca'='angelca'
'angelca' going around 'i' is 'ANGELICA'.
(Other definitions for angelica that I've seen before include "Its candied leaves are used in cake decoration" , "Candied plant stalks" , "Candied leaf-stalks, used to decorate cakes" , "Candied plant stem" , "Girl" .)