Plant girl’s knocked over (5)
I believe the answer is:
senna
'plant' is the definition.
The answer is an example.
'girl's knocked over' is the wordplay.
'girl's' becomes 'anne's' (girl's name).
'knocked over' is a reversal indicator.
'annes' back-to-front is 'SENNA'.
(Other definitions for senna that I've seen before include "Its leaves are used medicinally" , "Anne's upset by a laxative" , "Brazilian F1 champion, d. 1994" , "Medicinal plant - racing driver, Ayrton" , "Cassia tree, laxative" .)