Plant left behind rag (6)
I believe the answer is:
Here is my best explanation:
'plant' is the definition.
(teasel is a kind of plant)
'left behind rag' is the subsidiary indication.
'left' becomes 'l' (common abbreviation).
'behind' is a charade indicator (letters next to each other).
'rag' becomes 'tease' (teasing is a kind of ragging**).
'l' after 'tease' is 'teasel'.
(Other definitions for teasel that I've seen before include "Herb with prickly flower heads -- elates (anag)", "Elates (anag.)", "Plant with spiny purple flower heads", "it helps us to nap", "Prickly wild plant", "Prickly plant (S, not Z)", "Prickly plant - elates (anag)", "Sat eel up on the herb", "Plant with prickly flower heads" and "Tall prickly plant".)