Bites back endless pain from plant (7)
I believe the answer is:
spinach
'plant' is the definition.
(spinach is a kind of plant)
'bites back endless pain' is the wordplay.
'bites back' becomes 'spin' (I can't justify this - if you can you should believe this answer much more).
'endless' means to remove the last letter.
'pain' becomes 'ache' (ache is a kind of pain).
'ache' with its final letter taken away is 'ach'.
'spin'+'ach'='SPINACH'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for spinach that I've seen before include "Blackboard stand" , "leaves on plate?" , "Vegetable leaves" , "something to eat" , "Popeye's vegetable" .)