Flatter round base of aquatic plant (9)
I believe the answer is:
buttercup
'plant' is the definition.
(buttercup is a kind of plant)
'flatter round base of aquatic' is the wordplay.
'flatter' becomes 'butterup' (buttering up is a kind of flattering).
'round' means one lot of letters goes inside another.
'base of' suggests the final letters.
The final letter of 'aquatic' is 'c'.
'butterup' enclosing 'c' is 'BUTTERCUP'.
(Other definitions for buttercup that I've seen before include "Bumboat woman" , "Weed with rich yellow flowers" , "Goldilocks, possibly" , "Dockside seller" , "Common meadow flower" .)