Medicinal plant cultivated in vigneron's empire (7,8)
I believe the answer is:
evening primrose
'medicinal plant' is the definition.
'cultivated in vigneron's empire' is the wordplay.
'cultivated in' indicates an anagram (cultivate can mean to polish or improve).
'vignerons'+'empire'='vigneronsempire'
'vigneronsempire' anagrammed gives 'EVENING-PRIMROSE'.
(Other definitions for evening primrose that I've seen before include "Herbal remedy" , "flower" , "flowering plant" , "Plant with pale yellow flowers" , "type of oil" .)