Flatter, taking in 100, in plant (9)
I believe the answer is:
buttercup
'plant' is the definition.
(buttercup is a kind of plant)
'flatter taking in 100' is the wordplay.
'flatter' becomes 'butterup' (buttering up is a kind of flattering).
'taking in' means one lot of letters goes inside another.
'100' becomes 'C' (Roman numeral).
'butterup' placed around 'c' is 'BUTTERCUP'.
'in' is the link.
(Other definitions for buttercup that I've seen before include "Rich yellow wild flower" , "Common yellow meadow plant" , "Dockside seller" , "Meadow flower poisonous to cattle" , "found in meadow" .)