Poison left to rise up in a wine unopened (8)
I believe the answer is:
atropine
'poison' is the definition.
(I've seen this before)
'left to rise up in a wine unopened' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'to rise up' is a reversal indicator (in a down clue, letters go up).
'in' indicates putting letters inside.
'unopened' means to remove the first letter.
'port' written backwards gives 'trop'.
'wine' with its initial letter taken away is 'ine'.
'a'+'ine'='aine'
'trop' put within 'aine' is 'ATROPINE'.
(Other definitions for atropine that I've seen before include "Poison found in deadly nightshade used as a drug" , "Drug obtained from deadly nightshade" , "Poisonous substance" , "A pointer (anag) - belladonna" .)