Drug left over stopping boundless hurt (8)
I believe the answer is:
atropine
'drug' is the definition.
(I've seen this before)
'left over stopping boundless hurt' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'over' shows that the letters should be reversed in order.
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'boundless hurt' becomes 'aine' (I can't justify this - if you can you should give a lot more credence to this answer).
'port' backwards is 'trop'.
'trop' placed into 'aine' is 'ATROPINE'.
Can you help me to learn more?
(Other definitions for atropine that I've seen before include "Drug obtained from deadly nightshade" , "A pointer (anag) - belladonna" , "Poison found in deadly nightshade used as a drug" , "Poisonous substance" .)