Poison pirate on waves (8)
I believe the answer is:
atropine
'poison' is the definition.
(I know that atropine is a type of poison)
'pirate on waves' is the wordplay.
'waves' indicates an anagram.
'pirate'+'on'='pirateon'
'pirateon' anagrammed gives 'ATROPINE'.
(Other definitions for atropine that I've seen before include "A pointer (anag) - belladonna" , "Drug obtained from deadly nightshade" , "Poisonous substance" , "Poison found in deadly nightshade used as a drug" .)