Plant, rush, by spring (9)
I believe the answer is:
speedwell
'plant' is the definition.
(speedwell is a kind of plant)
'rush by spring' is the wordplay.
'rush' becomes 'speed' (synonyms).
'by' says to put letters next to each other.
'spring' becomes 'well' (both can mean a source of water).
'speed'+'well'='SPEEDWELL'
(Other definitions for speedwell that I've seen before include "Flowering plant" , "Small plant, veronica" , "Small blue flower of the genus veronica" , "Small creeping herbaceous plant with blue or pink flowers" , "bloomer?" .)