To tantalise the left is a plant (6)
I believe the answer is:
'plant' is the definition.
(teasel is a kind of plant)
'to tantalise the left' is the wordplay.
'to tantalise' becomes 'tease' (teasing is a kind of tantalizing).
'the left' becomes 'l' (common abbreviation).
'is a' acts as a link.
(Other definitions for teasel that I've seen before include "Elates with nap-brushing device" , "Sat eel up on the herb" , "Eel sat up on the plant" , "Plant with prickly flower heads" , "Herb with prickly flower heads -- elates (anag)" .)