Rush, better plant (9)
I believe the answer is:
speedwell
'plant' is the definition.
(speedwell is a kind of plant)
'rush better' is the wordplay.
'rush' becomes 'speed' (synonyms).
'better' becomes 'well' (similar in meaning).
'speed'+'well'='SPEEDWELL'
(Other definitions for speedwell that I've seen before include "Small plant, veronica" , "Small creeping herbaceous plant with blue or pink flowers" , "bloomer?" , "Flowering plant" , "Small blue flower of the genus veronica" .)