Dock stalk on plant (9)
I believe the answer is:
penstemon
'plant' is the definition.
'dock stalk on' is the wordplay.
'dock' becomes 'pen' (I've seen this in other clues).
'stalk' becomes 'stem' (synonyms).
'pen'+'stem'+'on'='PENSTEMON'
(Other definitions for penstemon that I've seen before include "Step on men, strangely, to get to the flower" , "plant" , "showy flower" .)