Tall prickly plant (6)
I believe the answer is:
teasel
'tall prickly plant' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for teasel that I've seen before include "Plant with spiny purple flower heads" , "Plant with prickly flower head" , "Prickly plant (S, not Z)" , "Eel sat up on the plant" , "it helps us to nap" .)