Sailor pursues war over poisonous plant (8)
I believe the answer is:
mandrake
'poisonous plant' is the definition.
(plant of the nightshade family)
'sailor pursues war over' is the wordplay.
'sailor' becomes 'drake' (explorer Sir Francis Drake).
'pursues' says to put letters next to each other.
'war' becomes 'nam' (informal term for the Vietnam War).
'over' says the letters should be written backwards.
'nam' in reverse letter order is 'man'.
'drake' put after 'man' is 'MANDRAKE'.
(Other definitions for mandrake that I've seen before include "Poisonous plant with large forked root" , "Plant with medicinal properties" , "Exotic plant" , "May apple" , "Plant once believed to shriek when pulled up" .)