Small plant of the primrose family, with flat five-petalled flowers (sometimes scarlet) (9)
I believe the answer is:
pimpernel
(Other definitions for pimpernel that I've seen before include "The Scarlet ..., aka Sir Percy Blakeney" , "Bloomer" , "Baroness Orczy's scarlet flower?" , "(Scarlet) plant of the primrose family" , "rarely identified in France" .)