Plant left behind rag (6)
I believe the answer is:
teasel
'plant' is the definition.
(teasel is a kind of plant)
'left behind rag' is the wordplay.
'left' becomes 'l' (common abbreviation).
'behind' says to put letters next to each other.
'rag' becomes 'tease' (teasing is a kind of ragging**).
'l' put after 'tease' is 'TEASEL'.
(Other definitions for teasel that I've seen before include "Herb with prickly flower heads -- elates (anag)" , "Dried flower head" , "Plant with prickly flower heads" , "Prickly plant - elates (anag)" , "Sat Lee down on the spiny herb" .)