Plant donor leg set by doctor, initially (9)
I believe the answer is:
goldenrod
'plant' is the definition.
(goldenrod is a kind of plant)
'donor leg set by doctor initially' is the wordplay.
'set' is an anagram indicator (instruction to set the letters).
'by' is a charade indicator (letters next to each other).
'initially' says to take the initial letters.
The initial letter of 'doctor' is 'd'.
'donor'+'leg'='donorleg'
'donorleg' is an anagram of 'goldenro'.
'goldenro'+'d'='GOLDENROD'
(Other definitions for goldenrod that I've seen before include "Autumnal flower" , "Yellow-spiked garden flower" , "Autumnal plant" , "Yellow-headed garden plant" , "Plant with tall spikes of yellow flowers" .)